The problem was indeed the avbin backend, or more precisely the fact that the newest avbin version in Gentoo was very old. I have since bumped it. Now the sound is acceptable.
Having said that, even the newest avbin version from upstream is still quite ancient, as is the libav they bundle. Not least of all, their package seems a bit confused. As far as I have seen it’s just a .c and an .h file with lots of spaghetti code around them, and a build system that is reliant on git. long story short, I would advise against using it.
MovieStim2() works well with respect to playback, though it has its own issues.
I have commenced packaging the dependencies required for MovieStim3() for Gentoo, and in my testing environment it seems the playback is great. Smooth, good sound, and not having any of the aspect ratio issues I have seen with MovieStim2().