Well, some good ideas from my front, I hope you like them...
The sound FX issue. You only really have one viable alternative: Have AUDIO portal moderators. Someone to delete things when they find them to not be the creation of the person who uploads them. While this will not deter people to upload content that isn't there's, there is a method to do as such. A warning. In big bold letters:
EXPERIENCE POINTS WILL BE DEDUCTED IF THE MATERIAL YOU ARE UPLOADING IS FOUND NOT TO BE YOURS.
I think with this kind of disclaimer, it will cut down the amount of pirated content to an extreme extent. Plus, with a few 'audio mods', the more clean it can be.
The Second Issue:
.WAV files not previewing right. Yes, this definitely an issue that should be dealt with. Here is one thing you could do - Have the afforementioned Audio Mods convert the WAV to MP3's and replace the wav files. Or, institute an MP3 only mandate.
The Third Issue:
File size. People keep bitching about it. I think that if they have something that is over 500k and they really think it should be allowed, do what you do for FLASH movies, and have them be sent to you to see if it's acceptable to allow the overage. This is just an idea. :) I'm full of 'em.
Oh and you should have a FEATURED ARTIST section in there... oh wait... you do... oh, shoot, it's me! ;)