A little update from my side:
I was abe to downgrade glibc to 2.12.2 and my sound problem [1] is now gone again! If it's not glibc itself, it's one of the packages re-installed after (again, see [1] for the list). If anyone considers masking glibc 2.13 for now: please take my vote. Best, Sebastian [1] https://bugs.gentoo.org/show_bug.cgi?id=354395
