Alright, I already know about SVN's being somewhat unstable. However, I need a decent workaround for this problem:

I have all the backends and such installed for Phonon with Xine and GStreamer. I have the correct Xine and GStreamer plugins, and Amarok at least works in 4.1.

However, I only get the occasional system bell as a sound in 4.2 SVN. What's worse, when I open up my Sound Preferences, there aren't any backends listed!

What should I do? Is the solution as simple as moving the 4.1 Phonon backends to 4.2? If so, what folders?

I truly love 4.2, but I need sound!

Are you compiling + installing the KDE 4.2 SVN Phonon backends? It sounds like you are missing some development headers and therefore they are not being compiled. I would not recommend copying the KDE 4.1 Phonon backends to KDE 4.2 SVN. they will more than likely be incompatible due to being linked with different Qt / KDE libraries.