With practice, you'll eventually be able to listen to a passage of music a few times, recognise what the relationships between pitches are, and then notate it straight onto manuscript paper, but this skill takes time to develop. To start with, use an instrument to play along with the audio files, to help you work out the music. Piano is ideal for this, but guitar is useful too. Single-line instruments are less useful for this (e.g. trumpet, clarinet, recorder), as you can't play more than one note at a time, and so can't work out chords as easily.
Take a slow and steady approach to working out musical passages. Your memory is a factor here too; just try to work out short passages of music, maybe just a few notes at a time. Any more and you will not be able to remember the whole passage:
- listen to the start of the audio file; stop the audio playback.
- use your instrument to find the first pitch of the passage (you may be able to do this while the audio is still playing).
- restart the audio file; listen to the direction and distance following notes move in; you can even do this one note at a time; stop the audio playback.
- having heard which direction (up or down) and distance (by step or by skip) the notes following the first note move by, try playing these notes on your instrument. Do they sound the same as the audio passage to you? If not, try other notes. (Listen to the audio file again if necessary.)
- Once you think you have found the notes, try playing them along with the audio file. Do what you are playing and the audio file sound the same? If not, start the process again!
- Once you are confident that you have found the right pitches, make a note of them, either with conventional notation or even with "letter names".
You may find this process easiest to do if both the audio file and instrument can be heard "out loud", i.e. without headphones; otherwise it may make sense to use headphones for the audio track and then taking them off when listening to the instrument (some people listen to the audio with one in-ear headphone in, and one ear free to hear the instrument - I don't like this much!)
As you become better at this process, you will find that you can play along with musical passages reasonably well, stop the audio file fairly sure you have played the right notes, and write them down straight away. But, realistically, to start with, it makes sense to always try playing the notes with the audio file stopped, after having listened to (or tried playing with) the audio file; it is hard to listen to both your own playing and the audio file at the same time.
You need to be able to easily find different parts of your audio file. MP3 players and CD players are reasonably good at this, as you can rewind to a particular time, but it is far easier to use computer software that allows you to see the audio wave-form. This makes it easier to find where you are in an audio file; you can usually set markers and even automatically repeat passages of the audio file.
Yes, it is harder to work out passages with chords, than single line melodies. But, you can still take the same approach as above. The trick is to try not to listen to all the notes in each chord at the same time. Firstly, focus upon the top notes of a passage; listen to the direction they move in. This is likely to be the melody part, similar to your first file (although this isn't always the case!) Once you have worked out part of the "top line", work out the lowest notes for the same passage. The highest and lowest pitches in chords are usually the easiest to hear, and so work out. Once you are fairly confident that you have correctly worked out the top and bottom notes of a passage, you can work out notes in the middle of chords in two ways: firstly, by listening (again!); secondly, by using some common-sense - unless you are working out atonal music or music with quite advanced harmony, the chords are likely (again this will not always be the case!) to make triads - if you know what the top and bottom notes are in a chord, you can have a good guess at what any middle notes may be, as they may complete triads (again, I must stress, this is not always going to be the case!)
To sum up:
- use an instrument to help you find the notes you hear in an audio file
- work out short passages, regularly stopping the audio to try out notes on your instrument, without the audio file playing
- work out passages with chords, by working out the top notes and then bottom notes first, before using some knowledge of probable chords to work out notes in between
- use software or hardware that makes it easy for you to jump to specific parts of your audio file
The process of listening to, and then notating music, is called transcription. There are various types of software available to help you transcribe music. Here are some suggestions: