Most current MPEG, which is MPEG layer 3, is proprietary, and therefore not
too accessable.  It also comes in a number of different implementations. 
However, in general, the data rate is adjustable, like JPEG, to different
quality levels.  Compression rates vary from 20:1 or more, which sound
horrible, to no compression, or even data expansion.  The standard settings
are usually 10:1 compression.  Personally, I find that the quality is
significantly degraded, despite the fact the everyone trumpets the idea that
MPEG-3 is "CD-Quality."  A lot of it depends on your playback system, since
much of the loss is in the high frequencies.  If you are listening on little
computer multimedia speakers, the max frequency is probably 12-15k.  So who
cares if the higher frequencies are lost.  But on my studio monitors, the
compressed copy rarely sounds too much like the original.
	MPEG layers 1 and 2 generally perform worse than MPEG layer 3.  There is
source around for 1 and 2, though, since they are not proprietary.
	On the stereo issue, a common trick is to simply make the compressed version
mono, therefore achieving an instant 2:1 compression.
	There are some demo apps around for MPEG-3 encoding, like MPecker.  There are
many more MPEG-3 decoders.
-- 
Mike Berry
mikeb@nmol.com
http://www.nmol.com/users/mikeb