best way to overlay audio tone?

[alex4801]

I have VLC streaming a police scanner. Audio comes in from the hardware audio-in jack. When listening to the stream from a remote location (e.g., a phone) it's difficult to know whether the stream gets disconnected because the audio is not constant, as is the nature of scanner traffic. What I'd like to do is overlay some sort of audio tone every 30 seconds, so that when listening to the streaming audio I know that as long as I'm hearing a tone every 30 seconds it's still connected. What would be the best way to go about this?

Thanks,
Alex