Libsndfile 1.0.13 has been out since January, and the next audacity release will depend on it. 1.0.12 ebuild works with the epatch line taken out.
Created attachment 82940 [details] working ebuild for libsndfile 1.0.13
Will take a bit, I've seen their configure.ac is totally foobar'd wrt to AC_ARG...
Okay I've bumped to 1.0.15, with a patch to fix AC_ARG_ENABLE and new alsa useflag. I've also sent the patch upstream, hopefully we can drop it for next releases.