The command description is described in the quick manual. (Press A when the cursor is over the command and look at text in the top row.)
Values 0-7 = absolute volume. Values 8-F are relative. And yes, even volume value zero won't make the volume go to complete silence. Try putting a note with the default instrument (the important part being, a non-changing volume envelope like A8). Then put a M77 command on the note to reset the volume. Then put MFF commands on the rest of the (empty) rows in that phrase. Or you could manually put the values from 7-0. Then play. This should give you an idea of the range the command has. This also differs between Gameboy models because the global attenuator (most likely) is an analog attenuator that is not perfect.

i've found that m commands dont seem to have any effect with values > 77
the thing is that volume changes with the M command are pretty subtle, and M00 isn't "zero volume". maybe you should just record it and either turn the volume pot down manually at the end or do the fadeout in post (like pretty much everyone else does! )

I'm triggering the commands in the noise channel to control the volume of the rest of the channels. Well that's what I'm trying to do lol

I'm using LSDJ on a DMG. There's only one M command at a time.

Are you using it on an emulator? Or on hardware..
It should work...

Are their multiple M commands that get triggered on the same time?

Well, initially I tried using M 77, M 66, ..all the way down to M 00. Then, I tried M FF over and over. Neither of these worked.

What values are you using?

I'm trying to use the M command in LSDJ to automate a volume fade out, but it doesn't seem to do anything at all. Am I using it incorrectly?? Or are there better ways to achieve a fade out?
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