audiohooks: Muting a hook can mute underlying frames

If an audiohook is placed on a channel that does not require transcoding,
muting that hook will cause the underlying frames to be muted as well.

The original patch is from David Woolley but I have modified slightly.

ASTERISK-21094 #close
Reported by: David Woolley
Patches:
      ASTERISK-21094-Patch-1.8-1.txt (license #5737) patch uploaded
      by David Woolley

Change-Id: Ib2b68c6283e227cbeb5fa478b2d0f625dae338ed