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  :P 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.