Banshee and Mono

Gstreamer is LGPL which is designed to allow this sort of usage.

I figure that in order to be safe from stepping on anybody's toes it is
important to be able to satisfy the licensing of all software that you have
shipping as a single product.

A plugin mechanism provides some level of abstraction, but I guess it is not
enough to prevent creating a derivative product by combining two otherwise
separate pieces of software. "Derivative" as used by a legal term to indicate
were your responsibilities for copyrights end and begin.

This sort of thing is why we have lawyers, I suppose.