Here be licensing dragons, folks, at a minimum when you go from one
codec to N codecs.

Be very careful on the selection of multimedia codec frameworks, as
they get you into licensing hades more often than not, and many people
don't see it coming.

Here's the problem:

You want to plug in a commercial licensed codec into a codec framework,
to get at a patented algorithm (of which there are many in the media
area, and software patents cannot, unfortunately, just be ignored,
despite most of our beliefs the current system is badly broken)  

But the license of the framework's codec interface has terms that
conflict with the commercial codec's license terms (typically around
patent issues).

Net result: no legal combination.

This may be ok from an end user's point of view when they put their
system together out of pieces, as they usually ignore the legal
problems, but it is a showstopper for re-distributors (e.g. Linux
distributions, OLPC downstream consumers), who might like/need things to
work "out of the box" for the end user.

As an example of lack of care about this is the Xine player libraries,
which would have been perfectly adequate several years before the
gstreamer library was built.  Gstreamer was explicitly written and care
taken in its licensing to allow for such combinations to be possible,
and arguably would not have been necessary had the licensing issue been
thought through in advance (it was infeasible to get Xine's libraries
re-licensed, due to the number of contributors).

I have no information about mplayer's licensing situation.

Once burned (actually, free software has been burned multiple times on
this topic), twice shy.  Please be *very* careful in this area so you
(and we) don't get burned too.
                                    Best regards,
                                       - Jim

-- 
Jim Gettys
One Laptop Per Child