If the audio file is already in MP3 format, I use AudioSlicer (http://audioslicer.sourceforge.net/), to split the file into multiple files. AudioSlicer will do this without lowering the quality by re-encoding the MP3 file. (Note: I do this on a Mac that runs MacOS 10.3.9. AudioSlicer 1.3 does not run properly on MacOS 10.4.6.)