Control/switching of headphone audio
My first thought was what about relays.  I've done a quick search and found this http://www.geofex.com/article_folders/relays/relays_for_switching_audio_signa.htm and this https://electronics.stackexchange.com/questions/110750/is-it-possible-to-turn-an-audio-signal-on-off-using-a-solid-state-relay so maybe it's not such a stupid idea, but like optocouplers it probably is a bit OTT.