I see you're using Directshow Output.
You may want to try, first, working through the
Audio Setup Guide in the Wiki before you proceed. Unless you're on Windows XP with junky hardware, you should probably be using WASAPI Event Style (or something other than Directshow).
I don't think that'll solve your Volume problem, but it should help get it into a "known good" state.
Also, check to be sure you don't have anything odd activated under the DSP Studio. If you have Volume Leveling, Room Correction, EQ, or any other filters enabled other than Output Format, try deactivating them and see if it fixes it.
If so, re-enable one at a time and figure out where you went wrong.
Otherwise, Adaptive Volume Light should increase the volume of AC3 sources to fill available dynamic range (they are often mixed very low). However, I should add, that I don't know if MC will do the same with DTS, DTS-MA, and Dolby TrueHD sources, so for those, you may indeed need to increase volume levels.
Also, yes, as Jim indicated, consider
what Volume mode to use. If you can deal with the side-effects, Internal is best. This is more difficult when you have a multi-function HTPC that will use other applications that depend on the System Volume control (mostly games and web browser stuff), but the internal Volume control in MC is more capable and higher quality.