The app-emulation/emul-linux-x86-medialibs-20130224-r13.ebuild only supports ffmpeg. I haven't tried yet, but the setup will end up using 32bit ffmpeg with 64bit libav for me, which doesn't seem reliable (compiling against libav headers but linking with ffmpeg binaries). Reproducible: Always Expected Results: 32bit libraries built by app-emulation/emul-linux-x86-medialibs-20130224-r13 should match those of the main system.
The 32-bit ffmpeg is for ffmpeg slot 0.10, which should be parallel-installable with ffmpeg-9.x. And there is no 0.10 slot of libav to use as an alternative. If you do encounter a problem with libav-9.9 and ffmpeg-0.10.8 on one system, please reopen a bug.