What I did was I searched my system for 'ac3', which happened to be located in the following directory C:\Program Files\K-Lite Codec Pack\filters (yes, it's through K-lite). Then I opened it and some properties window popped up. Then under gain, I increased the intensity from 0db to +20db for both Master and Gain and set the 'output' to the type of audio I wanted (surround, mono, stereo, etc - stereo for me) and then clicked 'apply' and 'OK' and then reopened the file in MPC/mPlayer. Of-course, I did put the master volume a bit up as well along with treble and bass level, and I was satisfied with the result.