voice activated circuit with static burst
Your question is very broad and probably won't stay open, so I'll be brief.  I'd take the audio in, scale/shift as needed to go into the A/D of a micro, do amplitude detection in the micro, low pass filter that, add some hysteresis to detect voice on/off, and a time delay to ride out short pauses.  The hash sound could be always added to the amplifier input, just being quiet most of the time.  The micro should be able to produce a hash sound easily enough.