I havent done any sound with GS yet, but technically speaking, a mono sound can still come out of both speakers if its balance is in the center, the difference with stereo is that you have something different coming out of each speaker so if a stereo sound comes out of only one speaker, its not necessarily coming out mono the problem could lie somewhere else. Have you tried playing mono wavs? do they come out of both speakers? what if you pan(balance) the stereo wav to the side that has no sound? have you tried it with other wavs?