If you are really worried about this, I recommend surgery. I used to have this Windows box at work that would always make sounds for no reason, even though the sound was muted. The solution was to open it up and physically disconnect the speaker. Never made a sound again. If you don't want your computer to be used as a listening device, first try adjusting your tinfoil hat. If you're still worried, open it up and rem…
> There. Now you just have to worry about the bugs the Agents put all over your house. Physical bugs are expensive. Malware is cheap.
Working in telecom was enlightening for me. There's a feature called "executive barge in" that pops up from time to time when shopping for PBX systems. Executive barge in allows a user with the appropriate rights to open an audio channel to any phone connected to the switch, bypassing the alerting phase. That is to say, the phone never rings; the audio channel just opens. Most systems provide some sort of brief alert tone, but this is entirely implementation based. There's nothing implicit about opening a channel that would require a tone.
All digital phone systems have the ability to implement a feature like this. Cell phones are digital phones. With old analog (POTS) phones, when the phone was "on hook", there was a physical change in the connection of the copper pairs. In modern phone systems on-hook/off-hook is just a software state. There is no physical difference. Opening an audio channel is a distinct event, completely separate from the alerting signal [1] in common cell phone protocols.
The bottom line is that if you're really concerned that someone is listening in, you should watch the horrible movie "RED" and imitate John Malkovich's character the best you can.