AC3 encoded movies usually have very low volume. To fix this edit the /usr/share/xbmc/system/advancedsettings.xml file and add this inside the <advancedsettings>...</advancedsettings> tags:
<advancedsettings>
...
<audio>
<ac3downmixgain>48.0</ac3downmixgain>
</audio>
</advancedsettings>
Change the value until it fits the volume you want and beware because althougt the option says ac3downmixgain it boosts the volume for all codecs.
Ref: http://forum.xbmc.org/showthread.php?tid=164929&pid=1421472#pid1421472