There are two GPL licenses the GPL and the LGPL.

If you want to incorporate (cut and paste code, or link to a library or etc etc) GPL code, then you must agree to release your code that uses the GPL code under the GPL license.

Some people like the GPL, but want to make useful libraries of code available for use by people who are not releasing their code under the GPL. Thus there is the LGPL. Under the LGPL you can link to LGPLed code without having to license your code under the GPL.

So, if the np decoder is LGPLed they could use it. If it is GPLed they cant.

Of course if the empeg software had a nice load-plugins-at-runtime interface, there would be nothing to stop end users compiling the GPLed code and placing on the player themselves.