So some details for those who have offered to help;
Hopefully the file attaches correctly, it's a trim from a larger audio file. As you can hear, Houston initiates contact with the A15 crew (Rover, this is Houston), there's a pause where we can hear the astronauts talking, then we hear, faintly, Houston again (Rover, this is Houston) and then Scott responds (Go ahead, Houston).
As far as I can tell, the second time we hear Houston is essentially an echo, we can hear Houston through the open mic's of the astronauts (after an appropriate delay). For support of this idea, we have how faint the second Houston contact is, and that when Houston first initiated contact it dominated the entire signal, you couldn't hear any background from the astronauts, which is what I would expect as the Houston signal is outbound, so the system needs to stop receiving to send (wasn't this the purpose behind the quindar tones?), and several times during the missions you can hear Houston in the background, repeated.
This occurs around mission time 146:15:18. In the Lunar Surface Journal, it's tagged under the Genesis Rock section. I did see a chapter listing for the Spacecraft Apollo 15 film but can't locate it at the moment, but I do recall a section also tagged with Genesis Rock. Essentially, I'm after 16 seconds of audio, or confirmation that the Spacecraft Film also contains the initial contact from Houston, followed by the fainter contact about 3-4 seconds later, then Scott's response.
For some extra disclosure, the person making this claim of a 0.9s delay between Houston and the A15 crew hasn't confirmed this is the actual audio he's referring to. I've posted the trimmed version, linked to the original and tried to get confirmation however they seem very opposed to providing direct answers that might lock them into a position. The only details we got was "Rover this is Houston...Go ahead Houston" and I can only find this occurring once during Apollo 15 (it also helps there is about a 0.9s gap between the second Rover, this is Houston and Scott's response, so as far as I can tell it's the right section).
Your help is appreciated.