Remove obsolete file

doc/audio.txt

-
-Audio Device Sharing on Linux
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-
-Some audio cards support hardware mixing. Such cards can be opened at the
-same time from different applications natively. If this is not the case,
-you will notice that is not possible to play music or process audio in
-other applications while running Blink.
-
-For audio cards that can be opened only once at a time, there are
-workarounds. More information about them can be found at:
-
-http://sipsimpleclient.com/wiki/SipAudioDeviceSharingLinux
-