I have similar issues and I believe "sound" is on the CS to do list according to their KB.

A fix for a sound problem that I experienced with another vendor's multi-engine a/c has also improved the C-130X but not completely eliminated the issues.  You may want to try it.

On XP (the process should be similar for Vista), open the Sound control panel then click the Advanced button on the Volume panel.  When that opens adjust the slider for the "Wave" setting to 50% then close that window and click "Apply" on the Sound control panel (then close the control panel).  After doing this you will need to increase your volume control on your speakers and you may need to increase the FSX volume setting for the ATC voice but overall the engines sounds should be improved.