According to wikipedia:

"ISO images are not able to store and recreate CD-Audio discs, due to the fact that CD-Audio discs do not use a computer file system. CD-Audio discs are formatted in tracks, and the track numbers, index points and CD time code are encoded into the lead-in of the CD-Audio disc, and these reference points are found throughout the CD-Audio sub-channel. To store an accurate copy of a CD-Audio disc, a different image format must be used, such as DDP or BIN/CUE."