As of libtwolame 0.4.0, 384 kbps is not accepted as a valid bitrate
for encoding mono audio and the maximum bitrate is now halved to 192
kbps to comply with the MP2 standard. Example error:

    twolame_init_params(): 384kbps is an invalid bitrate for mono encoding.

Adjust the default bitrate calculation to take this into account.
Signed-off-by: James Cowgill <jcowgill@debian.org>