The manual is confusing, but it is not wrong. Your file (produced by AUDIO.RECORD) really is a 3GP, even if you disguise its extension as MP3...
Currently Run.java defines one only output format:
What the manual says exactly is "The file must have the extension .3GP. "
Because the file that is produced IS a 3gp.
So the manual better say "The file produced by this command is a 3GP, so you should give it a .3gp extension so that it matches its content."