ffmpeg contains simd code which is incompatible with musl
This is an upstream issue for ffmpeg.
They are now aware of the problem but it would not hurt for somebody who can speak for Alpine to create such an issue in their trac.
In practice, ffmpeg on Alpine-i386 works for some input/output data-and-settings combinations and segfaults for some other ones. Theora/vp3 decoder is broken in this way but possibly something else too.
(This is the same kind of problem as there was in libtheora, but in other code)
(from redmine: issue id 6252, created on 2016-10-02)