Use -q/-b instead of -qscale/-ab

-q is a concise alias for -qscale
I have seen no documentation for -ab anywhere, ~~and it doesn't work (at least for ffmpeg)~~ ok, it works. But it's still undocumented.
-b seems to be what we're looking for, based on [this](http://trac.ffmpeg.org/wiki/Encoding%20VBR%20(Variable%20Bit%20Rate)%20mp3%20audio).

The :a selects the audio stream - I don't know if this is redundant, but it seems safe and is used on that page, so might as well.