when you write an Audio CD, the audio tracks are encoded in a different way than when writing data tracks on a Data CD.
For data a better error correction (CRC redundancy) is needed than for audio - imagine the consequences if just a single bit is changed in a program file burned to CD, the whole program could be useless!
For audio the consequences of small errors are much less severe, because they will only produce a small and brief change in the sound - which you might not even be able to hear.
On a 700 MB (80 min) CD you can usually fit 703MB of Data or 806MB of Audio; without overburning that is. If your drive supports overburning (most modern drives do), you would normally be able to burn an additional 1-2 minutes of data/audio depending on the media (CD-R) you use. You can test your drive’s and your media’s overburning capability with Nero CD-DVD Speed (and other programs also).
In order to overburn a CD (or DVD) with Nero, you first have to enable overburning in Nero preferences (under Expert Features), and even then you will be warned each time you attempt an overburn; in other words it is almost impossible to overburn “by accident” in Nero, but it can certainly be done.