the CDR has 80 minutes for an audio CD. That's about 80MB of mp3 files (128kB/sec sampling). If it's for a car CD player that can playback mp3 files, then you should make a data disc and it can take up to 700mb of mp3 files
I tried to make an Audio +Data cd using Nero express and still got the same error message as before ,i.e. not enough space on the disk ! Also I copied all the MP3 files to cd using explorer but my car's disk player says there is a disk error.