Use FFmpeg for audio compilation

This rewrites the audio compilation part of the content pipeline to use FFmpeg instead of NAudio / MonoMac's Audio Toolkit.

I've tested compilation of audio assets on Linux and sound effects playback successfully (although music is being converted to Ogg format instead of Wave, but that's a separate issue).  As I don't have access to any other platforms (only Windows in a VM with no Audio), I'll need other people to verify that the audio compilation works and results in XNB files that can be played back

I'm pretty sure the format names for FFmpeg are correct for the conversion, but I'm less sure about the muxer names (which are the wrappers around the encoded audio data).  For PCM, the muxer had to be "s16le" which means "raw PCM data"; I'm not sure whether the ADPCM or IMA ADPCM formats should be using the Wave muxer or just contain the raw data.