IIRC there's some problem when burning mp3 files as CD Audio with early versions of Nero 7, and this may have been fixed in later versions.
You could also try the small, simple and free Burrrn
I also second Ian's suggestions of trying other CD-R media and burning at a lower speed. Burning at 16x is usually a good choice for burning CD Audio.
Pioneer DVR-112L, LiteOn SHM-165P6S, Pioneer DVR-111L, BenQ DW1655, Plextor PX-760A
LiteOn LH-20A1P, Optiarc AD-7203A, Matshita UJ-840S, Sony RDR-HX910, Sony RDR-HX750