I don't know if this is a bug or should be a feature request, but it looks like make it one doesn't support low bit rates. I tried to join some files encoded at 48/32/mono and was prompted to select a bitrate. I tried 96 just for kicks as 48 wasn't on the list but ended up with an unusable file. It played but at a much faster speed than should be.
This program would be perfect if it supported low bit rates.