There are some variations on MP3s. My guess is that the ones that you can't burn are Variable Bit Rate MP3s - something that was added later.

If your software cannot understand the MP3s, you will have to convert them to WAVs (16bit, stereo, 44.1KHz) yourself.

You can do this using WinAmp (www.winamp.com) which is free and has an option for outputting directly to disk rather than audio (look under options|preferences|Output).

Mark