I have just released the first version of SilentJack:
SilentJack is a silence/dead air detector for the Jack Audio
Connection Kit. It monitors the peak levels on a single JACK input
port, and checks to see if they are below a specified theshold.
SilentJack then runs a command after silence has been detected for a
given number of seconds. It then waits for the command the finish,
and waits for a grace period before detecting silence again.
Hope it is of use to someone!