Guys, I'm just laughing my a off again (I did this quite often in the last few days, makes me worry), because I can offer you a very unconventional and "rough" workaround for getting the sounds on the 727 on the very first loading.
Downsides? Well, you have to own the 707 too. That's no downside! This is another great plane!
I haven't tested stability and such things, but it works so far.

HOW ITS DONE

1) surf to the \Gauges directory of FSX, look for Captain_Sim.p727.sound.gau there and make a backup of it by renaming or something.
2) now surf to \SimObjects\Airplanes\CS_B707-300\panel, look for Captain_Sim.x707.sound.gau and copy it to the \Gauges directory of FSX.
3) now rename that Captain_Sim.x707.sound.gau in the \Gauges directory to Captain_Sim.p727.sound.gau
4) done!

Load the 727, FSX might ask for permission on loading that one gauge. Allow it!

You will see some error messages on top of your screen for some seconds, stating that some sound IDs are already there. Don't care about them, they only last some seconds and your sounds are working now!  8-)

Give some feedback in this thread if problems arise. As said, I haven't tested much and maybe this isn't a stable solution at all.  :-/
If I get some "working!" replies here in the thread, I'll setup a ticket for this workaround.

[edit]Works fine on my system. Those few seconds of error messages (it's shown like ATC text, so no big deal) soon pass by and you can enjoy the 707 click sounds (and windows for example) all the way, on the first loading of the ac.
I've just had a complete flight and didn't experience any problems.[/edit]