pic-ker, There is no easy way to add a squelch circuit unless the receiver you are using has an RSSI output available. This stands for "Reletive Signal Strength Indicator" and it is an output usually derived from the IF portion of the receiver. If this analog signal was to be compared to a reference voltage that signal strength would be the level that the signal has to be to (open or unsquelch or unmute) the the data stream.

Dave Purola,
N8NTA