Export: wav vs mp3

Why do the mp3's sound different than the wav files when exported? The mp3 files seem weaker and more hollow. Is there a way to fix this?

It could also be the means by which you're playing back the MP3s versus the wave files. Import your MP3 back into the same SONAR project where you can A/B it against the uncompressed playback and verify that there really is a difference. You may be surprised to find that there is not. If that's the case, then you're experiencing issues with your playback system, e.g. Windows Media Player, which has some well-known problems such as unexpected sample rate conversion.

But if you do find that the imported MP3 sounds noticeably different from your project during playback, when played within SONAR, then it may simply be the encoder quality settings, as Jonesey suggested above. If you're doing the encoding straight from SONAR, check the settings and make sure you're not defaulting to 128kb/s.