Fixed strange sound loading issue on MacOS

Sometimes (Not sure why!) SoundEffect buffer reports -1 ChannelsPerFrame, and treats it as stereo even being a mono sound. This results in a sound played at twice the speed, very high pitched.

It can happen to different sounds on two consecutive runs of the game, so it's pretty random right now.

Since on XNA all sounds had to be Mono (Or so I think), treating this case as Mono "fixes" the issue.