You can get the 256k audio (and other dash formats) with youtube-dl by using the experimental option:
Code:
--youtube-include-dash-manifest
And youtube-dl can (also experimental) use ffmpeg for automatic muxing of seperate video and audio streams with:
so you get:
Code:
youtube-dl --youtube-include-dash-manifest -f 137+141 "http://www.youtube.com/watch?v=qPK67Px8sR8"