Add support for 8.1 PCM audio

Add support for 8.1 PCM audio.

I get these on VLC 1.2.0 - 20110607-1635:

araw error: bad channels count (1-8): 9
avcodec warning: invalid audio properties channels count 9, sample rate 44100
avi warning: cannot get packet header, track disabled

araw error: bad channels count (1-8): 9
avcodec warning: invalid audio properties channels count 9, sample rate 44100
mkv warning: find KaxCues FIXME
mkv warning: cannot get block EOF?
main error: ES_OUT_RESET_PCR called

araw error: bad channels count (1-8): 9
avcodec warning: invalid audio properties channels count 9, sample rate 44100

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