One thing is that the CD's are usually restricted to 80 minutes. If you have 20 songs at 4 minutes each you are full of capacity. Now if you were burning a data CD you could use the full 700MB.
As for the other music format. Most of those are using .MP3 format, unlike the typical .wav formats you burn for audio CD's. If you put an MP3 in your car you could probably put 110-120 MP3's on a disk.