Why does everyone think ogg's have to be used? I don't understand.
Anyway, to clear things up:
ofp supports .wav, .ogg and .wss sound formats. BIS recommends using .wss unless you have a reason to use .ogg, because .wss performs better (less compression). I personally like ogg because you can customise how much you compress the sound. The reason .wav generally isn't used is because the files are uncompressed, and much, much larger. A 5mb mission would be like 15+mb if it used wav instead of ogg or wss sounds. But I also have run into the problem with .ogg files not playing if they are too small, and in these cases, .wss or .wav files are useful.
Now, as far as stereo and mono goes:
music and ambient sounds can be stereo. Radio, "playsound" and "say" sounds have to be mono (could be wrong about the playsound though...). I don't know the quality off the top of my head, but there are also different max kbps rates (sound quality) that each group can use.
Ok, I hope that clears up all this confusion....