Reenable simple audio.

Reenable simple audio.

Now that most of the multichannel work has been done, we can put the
wraps back on and reenable the simple audio model from 1.3.x, which is
less flexible but also easier for the casual user. Users can choose
between the old model (simple) and the new (multichannel) through a
UI toggle, or with a command-line flag; they map to the same processing
code.