When the sound suddenly stops working, did you make any changes before it stopped working?
If not, try this suggestion
You can try a different audio backend in Dolphin (Config -> Audio tab) and see if any of them improve the sound quality. Also, FPS below 60 will usually cause audio issues which become worse as FPS lowers.
And if that fails try reverting to a default smashbros_sound.brsar (if it has been edited) and clear the SD card's sfx folder.
If none of those things make the sound issues go away your audio driver may not be compatible with Dolphin. Try updating your audio drivers, that is the last idea I can think of.