I've had that happen a few times, I don't think it's anything serious. As far as I can figure, it's caused by a loose connection from the system to the cartridge, so unfortunately the only way to fix it is to turn off the power, take out the cartridge, and re-insert it. Usually once the audio begins to mess up, the game will freeze the next time it loads a new screen / area.
When cleaning, be sure to also clean the system's cartridge connector too. I'd recommend a microfiber cloth for that, as something like tissue paper can get snagged and leave behind residue.