For NES and SNES it might not be so hard *IF* you know some ASM for each system. You could just simply write a blank NSF for NES games and replace the exsisting one.
That may only work sometimes. A lot of games use the music engine for sound effects (the sound effects can often be found in the NSFs, even). Replacing the in-game music driver with a blank NSF will more than likely just mute all the sound completely.
Does music usually have its own sound channel that could be turned off/disabled? Or do NES/SNES/etc games not work that way?
Some very very old NES games dedicate individual sound channels to soundeffects. But they can do this because they pretty much lack any sort of respectable BGM. See
Ice Climber: the "music" is a simple "do do do do dooooo" played by the Triangle (only 1 sound channel), leaving both squares and the noise available for sound effect use.
Most games make use of all 4 of the NES's primary sound channels (there are actually 5, but the 5th requires a lot of PRG space, so many games don't use it at all) for BGM and have to "borrow" those channels for sound effects, muting that channel in the music just as long as the sound effect is playing -- then resuming the channel in the BGM.
SNES has more channels (8, I believe), but I believe the concept is the same. Games might have reserved maybe 1... or maybe even 2 channels for sound effects -- however I find it much more likely that they used all 8 in the music and did the same "sharing" thing that NES era games did.
Ditto for Genesis -- although I don't know how many channels the Genesis had (8 melodic + 3 rhythmic? something like that?)
I didn't know much how the NES works, but I do understand a bit about SNES and I can tell you that I've already learned enough to have the music in several games (over
play silence. Because every SNES/SFC game that I've looked at has a dedicated "silence" song track. Just repoint all of the spc songs to that byte (usually 00) and you're done.
Quite a few SNES games actually use one or two channels for the sound effects. Sometimes these channels are shared with the music, but sometimes they are isolated. For those games, you can easily achieve what you want by turning the music sound channels off and leaving the sound effects channels on.
This is true, take Romancing SaGa 3 for instance, and the "Magical Tank Battle" 2 of the channels are dedicated for playing the machine noise--nute those two channels or reduce its volume to 0 and you'll get the song without that annoying noise. Take the wind from Final Fantasy 6, in a couple of songs its also dedicated on 2 channels.
If he's wanting to record the sound effects for some reason he could just find the spc song pointers and repoint them to the silence track--however if the game he's wanting to do that do doesn't have a silence track then he's going to need to disassemble the spc core. Either way, its not easy for someone who doesn't know ASM. The first thing I'd do is see if there is a GG or PAR code fo the game in question to silence the music.
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Could you please write a small tutorial on how to do this for at least one game?