I used the above music file and had no issues.
Although the bitrate flutters in Audacity from the original source, Exporting as MP3 to a different filename and location, then converting via MP3toBMU does not generate any "Bit rate is not constant - variable bit rate reported".
Reviewing the new saved MP3, gives me a consistent 128kbps/44kHz during audio playback. The BMU also remains consistent at 128kbps/44kHz.
When music is available as a selection in the toolset, but won't play, there are three reasons.
1 - the .bmu is not in your /music directory.
2 - the .bmu name does not match that of the .2da.
3 - bitrate incompatibility.
Review your MP3 Export Setup under Audacity Preferences - File Formats. Force the bitrate to 128 via the options.
Force the sample rate to 44100khZ as well under Audacity Preferences - Quality
I'd like to say that should do it, but please report back with your results.
Thanks,
FP!