The error "Inappropriate audio coding in file" can appear when trying to burn CDs in Infrarecorder if one or more MP3 files in the project is not in correct CD-DA format ( http://en.wikipedia.org/wiki/Compact_Disc_Digital_Audio ).

The MP3 may have a sample rate other than 44.1KHz, or it may be in mono.
Fixing these 2 issues is easy using the free, and excellent, audio editor Audacity (you will need to grab the "Lame" plug-in from there also).
Once you import your MP3 file, you can see immediately what sample rate it is and whether it is in mono.

 To fix an incorrect sample rate, just change the project sample rate in the bottom left corner to 44100. An export to MP3 should then resample to this rate.

To change a track from mono to stereo:
1. click + drag across the visual waveform display to select the whole track.
2. Copy this (edit > copy, or Ctrl + C).
3. Go Project > New Audio Track.
4. Click anywhere in the blank waveform area of the new track and paste (Edit > Paste, or Ctrl + V)
5. In the small drop-down menu on the first track (tiny arrow near the top left), choose "Make Stereo Track".
6. File > Export as MP3