I don't have that problem with the sound on mine. My problem is that the sound fades in and out a lot.

I bought a payware C130 sound set: Turboprop Simulations - Lockheed C-130 Hercules Sound Pack FSX (AUD$14.05) which is very nice. I know that you probably don't want to spend extra money on a sound set, so you might want to try this free set I found at Simviation.com: http://www.simviation.com/1/browse-FS+Sounds-107-8, scroll down until you see C-130 Lockheed Hercules (it was the sixth one down) on that page in the FS Sounds section.

Mark