how to split an mp3 file into multiple audio CDs?

Your question is not specific or detailed enough to give a specific answer..

Is the file more than 700 Mb?

If not it will fit a standard 700 Mb capacity disk.

This free app, Deepburner Free, gives the option of choosing the number of disks to burn.

When the program opens "create data CD" is the default. This is what you need.

http://www.deepburner.com/?r=download

If this app doesn't work on Win 7 (I use it but on XP) try this one - CDBurner XP:

http://filehippo.com/download_cdburnerxp/

Your use of the term "audio CD" is also confusing.

The generic meaning of audio CD is one burned in wav format. In this format it will play on any CD player, new or old. Wav files are much larger. You don't say whether your file is music or speech but playing time can be used to estimate how many music tracks or length of track which will fit a 700 Mb / 80 minute disk.

An mp3 is a data disk and file size is considered more than play time.

About 18 - 20 music tracks in wav format will fit a 700 Mb / 80 Minute CD.

CD-R is the optimal type of blank disk to use.

To prevent errors during the burn process of wav format disks limit the burn speed to no more than 24x.

To burn as an audio (wav format) CD your mp3 file will need to be cut into smaller sections or songs.

This can be done with Audacity, a free audio recording / editing program.

Each section or song is highlighted by clicking, holding and dragging, copied and pasted into a new track then exported via the file menu and saved as a wav file.

http://audacity.sourceforge.net/

Basic tutorial:

https://wiki.brown.edu/confluence/display/CISDOC/A...

You have to burn the same file on the multiple CDs.

Try loading it into iTunes, right click the track, go to "Get Info", under "Options" select the time limit on the beginning and the end of your first segment. Then close the "Get Info" window, go to "Advanced" tab at the top, next to "Help". Select "Create MP3 Version". Then check your local music folder for iTunes. By the way, you might have to adjust your bit rates in the "Edit" tab- "Preferences" to make the segment files of lesser size.