As I wrote, the FFmpeg-based a52 plugin for ALSA is useless with software that uses FFmpeg (libavcodec) as a shared library, like xine and VLC, because libavcodec’s functions are not thread-safe.
As an alternative, Devils-Hawk published on ALSA’s tracker a new a52 plugin that uses aften instead. This version seems to be actually faster, and does not break with xine, not the same way at least. There is still a problem that Julian Scheel reported on xine-devel last week. I’ll see to investigate it tomorrow.
I’ve added the ebuild for this in my overlay with a simple CMakeLists.txt file to use with cmake (as it’s also needed for aften, it’s not an actual extra dependency), it should work, in theory, but it’s still unusable with PulseAudio too: it crashes down miserably when starting, and I can’t really see why, as it is.
Anyway, I’ll see to work on that in the next days, in my spare time.