You could look at making some small changes to the sound.ini file found in the FSX\SimObjects\Airplanes\CS_B707-300\sound folder (back it up first, of course).

I'm very happy with the sound of the CS707-300 and am not really worried if it isn't exact. It's close enough for me. With all my extra payware add-on aircraft telling me to use different FSX sound settings for each of them, I ignore them and settle for a setting that I like across the range. Although I had edited some of the sound.ini files a long time ago, I can no longer be bothered, I would rather be flying them. ;)

Mark