Well, don't be so fast with installation. Reggae is not fully compatible with earlier MOS Media. So all two apps (SeventhSense and Weather) using it will stop working and even may be unstable. No application uses Reggae currently. The page is targetted mainly for programmers, so they can learn and experiment a bit.

Multimedia68k.library is for programs compiled for m68k processor and running on MorphOS emulated. This mainly concerns applications written in AmigaE. Without this small library Reggae is not accessible from m68k code. Native PowerPC apps do not care about this library. You can safely install it together with classes, m68k applications (if any, but as I wrote the lib on request, I guess we may expect some) will use it, PPC apps will ignore it.