Thanks interpreter

In state it's not really possible and not intended in lib. Workarounds needed, for example:
-Plug "something" into headphones to mute/unmute internal speakers.

-Listen to both internal speakers and SPDIF out when needed, there's no delay at all in PCM (that could lead to some "echo").
The only mandatory condition for lib to set SPDIF volume is speaker set to internal (or headphones, that doesn't prevent SPDIF output, of course). Gotta think of killing that condition if possible
