What are some of the best pieces for me to practice transcription, given that I can already easily transcribe melodies, and with some difficulty can recognize simple chords in isolation, but find it almost impossible to discern chords in pieces?
Use songs that you like. If you like the music you are more likely to stay motivated and stick with it. If a song is too hard (as in you get stuck quickly) just try to find a song that may be easier. Something with a slower tempo, simpler instrumentation, and simpler sounding chords. If you are stuck move to a different song. If the style of music is just too complex maybe try going with a simpler style that is related. For example if i am into Bluegrass with all its super fast changes, maybe i would try to start with folk music which may have similar chord changes at slower tempos.
Once you pick a song here is a step by step approach that may work for you. Do as many steps as you can. If a step is too easy do it while doing the next step. If you can't do a step skip it. Use your knowledge of music theory to fill in some gaps. Don't drive yourself crazy, recognize your weak spots and move on. Come back to those later, or try to find a simpler song.
- Hear where each chord changes. Mark out the measures on a piece of paper, and put a little mark to notate where the chords are changing. Note the meter and tempo if you want to be complete.
- Try to figure out the key, notate it. If the key changes through the tune note that as well.
- If you can hear any of the chords off the bat fill those in, for instance if you hear the song ends on the I chord write it in. If you can identify that chord elsewhere in song, write those down.
- Find what you think is the root of each chord. If you can dictate/notate melodies this might be doable. Again, write those down. (Edit: If you can't figure out the root just find any chord tone i.e. a note that sounds stable over the chord. Just doing this will reduce the amount of chord choices that are available. For example if you find a stable tone that is an F, then find the chords that have F as a chord tone [D Min, F, F Min, B Min, G7, etc] . If you know the key find the chords that are in the key AND have that chord tone)
- Once you have a root and a key, try and figure out the major/minor flavor of each chord. If you play an instrument that can play chords (piano, guitar, etc) get it out, play a major chord at that root and see if sounds correct, play the minor, etc... at this point you may discover you have a root incorrect from a previous step. Fix it OR if you if you just know if it is wrong mark with a question mark. If you know the key and some music theory, this may be a time to use some music theory to know what chords are common in that key. Also try to think about common progressions, common resolutions, etc. Use this knowledge to support your ear.
- If the chord is more complex then a triad try to figure out what kind of chord it is. If you aren't sure just note that it is not a triad. Try to replicate the sound on your instrument.
The nice thing about writing down as much as you can is that you can come back and fill in the missing pieces as you get some experience and as your ear develops. Also you will collect a song book of your favorite songs.
some other tips:
- Sing while trying to figure things out. If you are just guessing it will be harder and you won't develop your ear as quickly. So sing the roots, and then try to sing a major 3rd and minor 3rd above that to see which sounds correct. Or sing the correct 3rd and then figure out if it is a major or minor 3rd.
- Do this on a regular basis. You wouldn't go to the gym once a month to try and get in shape. Same goes here. A small session every day will be better than trying to run a marathon once a month.
- If you don't know music theory, learn some. This will help you know what is common and expected. Also if you understand what you are hearing it will help you hear a similar thing in a different situation as well as help you remember it. This will also help you put names to the sounds reinforcing what is going on in your ear.
I've suggested in another answer pertaining to internalizing chord progressions that singing the arpeggios of a tune/song/excerpt/etc will really help the chord tones pop out at you when you hear them "in the wild." Sitting down with a lead sheet of roman numerals instead of outright chord symbols and attempting to sing their arpeggios is also a good idea as it focuses on the qualities of the chords as opposed to the note names (I'm a huge proponent of moveable-do solfege).
A more advanced ear training method I have worked extensively with and also taught was one developed/taught by the great Jazz pedagogue Charlie Banacos and focuses on identifying clusters of notes. You will need a partner or an ear training app that produces clusters to do this, but I'll explain the partner method. 1) Have your partner sit at a piano so you cannot see the keyboard. 2) Your partner will play a reference note (eg, middle C) and then will play 1 random note. Name the note. Do this several times until you are comfortable. Now, move to 2 simultaneous random notes after the reference note. Then three notes, four notes, and so on. (If you really want to crank up the heat, you can do several clusters in a row without a reference note; this will really test your musical memory!) Don't rush, try to hear "inside" the chord. This might seem like a silly exercise but the point is to hear "inside" a voicing and hear all the dissonances and consonances in it. Once you start to hear this, you will not only hear what the chord's quality is (minor/major/etc) at the drop of a hat, you will also be able to tell its inversion and any other characteristics much more easily. The most important part in my opinion is exposure, so if you can sit down with a lot of these sounds and familiarize yourself with them, you will start to spot them naturally.