Properly implement the 4 GB audio channels

[DaveTCode]

#11 was originally raised for this, now that the emulator is mostly complete and runs at 60fps in release mode it's worth considering adding the audio channels in.

#85 covers the 2 DMA based audio channels and this issue covers the 4 GB channels until they're worth separating out.

I did implement these channels on my GB emulator but they never sounded quite right so it's likely I did it wrong and will be starting again here.

It's also probably that this is a good fit for a scheduler and so I might block this behind #48