I usually use Audacity (free music program) to do my audio conversion, as I seem to recall it being mentioned somewhere that C2 won't convert if you're using Windows XP/Vista. So you could try converting the MP3's in Audacity (I think audacity requires an plug-in to handle the conversion) into both M4a and Ogg, then importing them into C2.
MP3s aren't transcoded to OGG, and there are a huge number of possible WAV formats - e.g. it's possible for a WAV file to actually contain MP3 data still, so the same thing applies. You should only import 16-bit PCM WAV files, preferably at 44.1KHz, those should always successfully encode to both AAC and Ogg.