This is a reminder in case you want to edit faster a tune, because the built-in editor doesn't allow to insert patterns for example, or for entering a track (sfx) like the 35th, you have to click 35 times... so I find it easier to work from an external text editor. (I don't think you need to edit the sfx outside the built in editor because it's already good enough)

Probably most people would have figured it out themselves, but for the music structure in the .p8 files, they are presented this way (it's the part after "music"):

A line with

00 41424344

indicates it's not enabled in the player (empty pattern).

Lines are presented this way:

aa (space)bbccddee

aa (00 by default) indicates the play status. If it's 00 it will play. With 02 it will loop. 01 indicates loop start. 04 is for stop. 05 is loop + stop (I don't know if it's used, maybe it will loop one time then stop)

bb is for first channel, cc is for the second, dd for the 3rd and ee for the 4th.
It's in hexadecimal. So 01 means first pattern in the sfx list. 0a is 10th and 0f is 16th.

41 means the pattern is not activated but if you activate it, you'll get 1st pattern and so on. (46 means the pattern is not activated and when you activate it, you'll get 6th pattern). After f (16th pattern, you increase the decimals, so 51 is for a non activated patter, which has 17th pattern behind. (40+11 in hexa).

I hope it helps!

see also "Data structures for sfx and music" : https://www.lexaloffle.com/bbs/?tid=2341 (more for programmers I guess)