Note an audio CD can only accept about 70 minutes of music, maybe your mp3 files add up to more than 70 minutes, that may cause the problem
Apart from that not sure why to only gets to 99%.
How long have you waited at 99% before giving up?
It may be checking the CD so leave it at 99% and go and watch a TV program and see if it is still like that at the end.
Note if you are putting the mp3 files on CD as mp3 files then the 70 minute limit does not apply, only if you are burning AUDIO CDs.