The audio detector you want is what is called vox. There are a lot available on the net (search for "vox circuit" in google).
That vox circuit basically keeps its output HI as long as there's audio at its input. There is an adjustement (by varying a capacitor value at design time) that tells how long a silence must be before resetting the output. This prevent pauses in voice to be interpreted as end of audio signal.

Then, you could use something called an edge detector (search for edge detector in that pdf file).
What that does it create a pulse (user definable length, also by changing a cap value during design) at both beginning and end of a input signal. You could then feed that to the empeg input. I'd suggest to use an optocoupler to electrically isolate both the circuit and the empeg, just to be sure no harmful voltage is sent to the empeg, or ground loops.

Code:

---

Audio Signal:
--/|/\/\/|--/|/|/|\/\/\/\--------------
Vox output:
__--------------------------________
Edge detector output:
__-_________________________-_____ 

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Hope this helps