I'm a little confused on the concept of hearing music in your head. I dont think I really do that. Some people I know say they can hear specific instruments as they "listen". Do I not have the wiring, or is it less hearing and more trying to recreate?
You're absolutely right: not everyone can do this, although it's rare to have no 'inner ear' at all. Most people's inner ear improves as they receive musical training; as you're asking on this particular site, I'm guessing you've either had some or are keen to begin.
Don't let this worry you. You may find that you only think musically when you're actually playing or actively experimenting. There's also a distinct advantage you have: getting a tune stuck in my head can keep me awake for several hours!
This is how I see it... Imagine something you've seen often. Can you "see" it in your head? Maybe you don't get a very detailed image but you'll probably see something. And I don't mean it should look like the image is actually coming through your eyes, you just kind of remember the image; the shapes, colors, some details. Similarly you might be able to remember how something feels, like how does it feel to touch the doorknob, etc.
The same way you can remember how a piece of music sounds. If you've listened to a piece many, many times you might remember it pretty well and be able to "play" it in your head. You don't actually hear it, but remember the aural image, foreground melodies, rhythms, probably some harmonies etc. If that image is detailed enough, you can really "listen" to it. For example you can follow different lines than usual or concentrate on large scale structure or whatever. And yes, you can hear specific instruments. For example you don't only hear the melody, but you hear how the melody sounds, which includes the timbre of the instrument.
You can also, at least to some extent, change it, just like you can imagine something you have not actually seen. Or if you're really good you can imagine a completely new piece. You can in your mind create the instrumentation, harmonies, melodies, rhythms, dynamics, anything (maybe not in full detail if you're not Mozart).
Listening to music is something we learn. It is cultural. In order to hear a sound and be able to read its elements you need training, exposure to music and the way music is made. Sound is, for the most part, a two dimensional type of information -- the air pressure in your ears and the time --. Since the pressure goes up and down in time people call it a wave, the sound wave.
When instruments are played, the pressure in the air produced by them is added together. Like 2+9, which gives 11. In principle, from 11 you cannot recover that it was 2 and 9 the numbers you added. It could have been 1+10. In the same way, from a given music, played by a violin and a piano for example, it is not determined that a piano and a violin were played together to produce that sound. For example, you can take, in a computer, the same wave and decompose it into the sum of two other waves (or more if you want), each of which doesn't have to correspond to the sound of a piano or a violin.
The reason we are able to make an educated guess that the sound comes from a piano and a violin is precisely that, education, experience. We have listened to pianos and violins and have learned (roughly) what sounds they can make. We humans are fairly good at pattern recognition and that is what we do. We detect rough patterns in the sound that reminds us of the sounds we have heard in past experiences.
This said. Even very experienced musicians can listen to the sound of an ensemble of many many instruments and miss detecting some of them if the music is complicated enough. Or at least it may take them listen to the sound many times.
If you lack the skill. That is not such a big problem. You can get it just by listening to music. Specially try some in which there are parts in which instruments play solo and then together. Even better if what they play solo is the same as what they play together. That way you build the memories of how the instrument sounds by itself and you can practice decomposing the sound when they play together into the sum of the different instruments.
Edit: I see. I mistook the "can hear specific instruments as they "listen"". Well, in that case, that "listen in your head" is most of the times using your memory of sounds you have heard and is far from the act of listening. On the other hand, there are also neurological conditions that make you actually hear sounds without external stimulus. In The man who mistook his wife for a hat, by Oliver Sacks, he talks about some cases like this. Many of us can also have small acoustic hallucinations, small rings, bangs, or buzzing in your ears (not coming from physical vibrations in your body like the blood flow). I personally have experienced some after long periods of sleep deprivation.
I've always heard music playing in my head, even as a little kid (I'm now in my late 60s). My parents had the radio on a lot and there was usually some music in the background at home, pop, jazz, blues, as well as some classical. My mother liked to sing along with the radio tunes. The Catholic school I went to was big on music and taught us choral singing - even taught us Gregorian chant! - and we listened to classical music once a week in class. My oldest brother played accordian well, and I learned a bit, and guitar too. With all that music around me, I always thought it normal to be able to not only hear music in my head at any time, away from any radio or record player, but also to expand on it, change it, make up new rhythms, chord progressions, and melodies, and so on, all in my head, with all the instruments playing their proper roles. I have learned to play some instruments, but none really well, so when I want to actually record a song I've created it takes me forever and a day to get it right on tape (or, nowadays, on the computer). I wish I had the hand coordination and dexterity to bring the music in my head to reality a lot easier. I like to think I've "created" some really great compositions, but they're still inside my skull! All that being said, sometimes my brain gets stuck on a tune and it drives me nuts playing over and over and over, especially if it's one I don't even like.
I don't hear music in my head at all... But when I write music, especially on a computer, I sort of hear instruments parts that are not actually here... I think it's a matter of practice, both in learning pieces and in composing yours. I think actually hearing music in one's head depends on an actual academic training : once you've learn rules of composition, you happends to hear what is supposed to come after following these rules...and then later on your musical path, you should or should not have a need to go beyond these rules.
The answer is really simple.
Good Sleep!!!!! Your mind's ear (inner ear) need to have good enough rest to be 100% when you want him to and sleep is also related to good memory. Have a good sleep habit and do those:
Listen to music.. I said LISTEN!!! NOT HEAR!! Listening requires mind's peace, good energy level, deep focus and repetitions. THIS IS THE GREATEST EAR TRAINING METHOD: LISTENING.. But the efforts will pay, may be as soon as day two if you do focus and repete the process. After you spent time listening to a song, or solo, or anything, it may even be only one instrument, at some time in the day, away from the music, you will hear it playing, a saxophone phrase or the drummer conclusion, or ever the singer's timber. The result is guaranteed...
Never over do it. 25 mins sessions is great!! Or you just stop when you're bored or tired. It won't enter your mind. Better practice this on 2 or 3 songs at a time to avoid the boring thing and get tired with a song you like.
Start to mimic the instrument you hear with your mouth. This has been the only ear training for many of the greatest musician you know. Imitation means learning... start low..... dont put yourself under pressure. Don't even try it the first few days, the confidence will come by itself... your ears will tell you ;)
Keep music in your life... listen often, as often as possible. Your mind will get tune to it, and the theory stuff will come alive, the secrets of music are in the recordings, not on youtube videos or tabs etc... YOUR EARS ARE YOUR BEST ASSETS AS A MUSICIAN, tune it to the music. The Focus listening process will do GIANT STEPS ;) to your musicianship.
Give it a try for a week, but JUST DO IT. Then you come back to me... You won't have the same opinion on music, everything will become so logic... That is.. do it and you will see.
Sorry for my bad english.. im not very good at it.