You have no control over that. The reason is because if you want to conver the WAV to MP3, then the file has to be on your server. But for the file to be on your server, it has to be uploaded, so either way, you're still uploading the WAV file. Therefore, the convertion has to be done before the upload, which you have no control over. You'll just have to ask your users nicely to convert their WAV files to MP3. Maybe point them to freeware programs that can do that and stuff...
Diego