Try converting your MP3s to WAV first. The bit rate on all audio tracks should be the same. No mixing 22.5kbps with 48kbps, vbr with cbr or otherwise. Then add them to your project.
You didn't mention your output container (avi, mpeg2, vob, wmv) or media (CD-R, DVD-data, DVD-video) These have different limitations and standards.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70