Is there a keyboard utility that lets you select which application for the MM keys? Perhaps an icon in the tasktray or in the Control Panel.
Are you using a HP MultiMedia keyboard or another MM keyboard?
Apparently HP uses a
non standard format for their MM-keys.
Maybe one of the following plug-ins...
HP Multimedia Button Support
Reproduction Control v1 36 (this review mentioned success with the plug-in)
RM-X for Winamp (this old post mentioned success with the plug-in)