split mp3 files to fit space on cd

For starters, unless you are only planning to backup the files, you cannot use standard file splitting or Zip software.
File splitters, or the latest versions of PK-Zip and Win-Zip will work fine for creating a backup spanning multiple CDs.

The file splitters all assume that you are going to connect the 2 halves back into a single file.

Zipping the files may actually make them larger, as MP3 is a already a compressed format.

That leaves using an audio editor, and splitting the files there.
You need either an MP3 editor, or to convert the files back to wave files and split them with a standard audio editor.

You can find freeware versions at www.nonags.com
Some freeware and a number of trial-ware programs at www.download.com
Chas