It's KINDA like having an old CD audio track on the disk like how Age of Empires I&II worked (Not quite the same thing but it's the best analogy.)
Instead of the game processing the music and voice files themselves and playing them back it has the Wii or GameCube do it for it. It just basically says "start playing audio from X part of the disk and tell me when you're done." so it can just sit back and worry about other parts of the game and not worry about it. Some games just decided to play their music the old fashioned way and others didn't.
The problem this makes with DM(L) is it's already faking that the disk is there when it's really on USB or SD so when the game tells the hardware "start playing music" the hardware's like "wait a second, there's no disk in there to play" so the best that has been done so far is to just have it ignore those requests or convince it not to ask in the first place which makes the game PLAYABLE but still kinda quiet.
What needs to be done is have the DM(L) take those requests ITSELF, reading and interpreting the music and voice information from the disk ISO IMAGE and mix it in with the other sound effects that are being played by the game the normal way.