Windows Media Player uses two different codec interfaces, whereas the Themes
code solely uses the Video For Windows interface.  The FhG MPEG Layer III
codec Windows Media Player uses is implemented as a DirectShow filter, so
it's not accessible by the Themes code.

You would at least need to acquire an ACM (Audio Compression Manager) MPEG
Layer III codec in order for this to work at all.  If you did get this codec
(one is installed when you install the Windows Media Tools, for example),
this still might not work, as I presume the Themes code solely supports .wav
files.