Microoptimization for hexString in Flash audio path

Turns out the old wisdom of building strings by putting them
in an array and join()ing them being faster than doing a bunch
of +=s on string objects is no longer true.

http://jsperf.com/join-concat/2

Reduces per-audio-buffer call overhead by a half a millisecond
or so on IE. :)
Microoptimization in audio decoder output copying

Use the Float32Array copy constructor on a .subarray() from the heap
instead of creating a fresh buffer initialized to zero then filling
it with data.
drop some unused vars
Microoptimization: pre-bind window.performance.now() to getTimestamp()

looking up 'window' 'performance' and 'now' on every call, a hojillion
times a second, wasted a teenie bit of time every time.
Microoptimization: more efficient loop on initial frame buffer setup

only run once but hey, it saves a few milliseconds
Clean up YCbCr loop a bit