In Demul, go to Config, then input. You will see a key that is set for "SERVICE" and "TEST" under System Board.
Press one of them and you will enter test/service mode:
Choose Game Test Mode (I took screen shots from MVC2 from my setup to help you out; it should work for any Naomi game)
Then go to Game Configuration. Simply change "SOUND MODE" to stereo here.
Regular Dreamcast games are already stereo (unless you downloaded a hacked CD image that got rid of stereo audio to fit on a CD-R or something).
Hope this helps.