Not sure how you had it set up, but the Sena works in full duplex just like a telephone conversation. There is no need to press an intercom button or to have "VOX" activate the mic for you. It's just on when you're connected to another one (be it your passenger or another rider). I haven't used the Sena with more than one other person, but it's been absolutely fantastic when I have. I use it daily for commuting to listen to music and answer phone calls. I can even send text messages by dictating to Siri with it.
You can only listen to music when the intercom is off. Therefore, you must activate the intercom every time when you want to talk and deactivate it when you want to return to music. That is what the VOX is supposed to do automatically. But it doesn't do it for us on the SMH-10. It triggers on wind noise all the time.
"Rumors of my assimilation are greatly exaggerated" - Capt. Picard.