Well, there's a sound.cfg in SimObjects\Airplanes\CS_B727-100\sound, but I don't know how to edit volumes there.
Otherwise, maybe some addon sound pack can help you. They are not that expensive and I've heard   good things about them so far. The JT8D-7-9A Soundpack should do it and can also be used on the 737 with those old cigar like engines.

I've found the original CS pack after 2.3 or so to be quite good though. But, yes, louder might be even more fun there.

Before you buy another pack, just search youtube for it and listen to the relationship of the volume when compared to other sounds. They usually crank those vids up in volume to impress everyone, but this doesn't necessarily reflect the experience when the things are installed.

Sometimes, those packs give you a too loud cockpit too. Remember, all your noisy stuff is in the back of the 727 plane and the wind is a strong player when flying, right at your windscreen.
I was a happy customer from quite some sound packs until I realized that, especially when simming with headphones, the noise is just too loud to be true in some cases, only impressive when flying in the spot view or so, but annoying when in the VC.
This depends from pack to pack though, so I can't generally say "go for it!" or "don't do it!" there.