I'm not the best at music to begin with, but I feel like 40-80 for sound of the 1024 compressed bytes seems like a good trade off even light 1k games use <900 bytes leaving room for music.  you would be limited to 3 of the 4 default tracks to leave room for sound effects.  

A simple tune twinkle twinkle using 2 channel uses about ~43 bytes:

?"\as∧l0wx5c1cccffc.ffccggc.ccffccg.ccffccg. \as∧l0wx5ccggaag.ffeeddc.ggffeed.ggffeed."

https://pico-8.fandom.com/wiki/P8SCII_Control_Codes#Audio_commands

the previous link gives an overview of using strings. one note is for the loop Lp0p1, they use 0-9 A-Z. A being 10 Z being 35. and you can only have a max of 32 notes. ex: l0w = loop notes 0 to 32

If you come up with something good I wouldn't mind a real composed tune, I used about 68-72 for spirit koi.