Rewrite audio processing to use ffmpeg

There's some fairly major changes here, including:

* All audio is resampled to 48kHz stereo on input, allowing files with
  non-standard or varying rates to be mixed
* The audio processing is now done in a single pass; ffmpeg reads all
  inputs, concatenates them, and outputs one file.
Merge pull request #373 from kepstin/recording-updates

Rewrite audio processing to use ffmpeg