I checked it out and yes the volume was low - and many others have encountered this.
Use this with caution:
-D or --drc: if your audio isn't loud enough, you can try applying extra dynamic range compression when encoding from AC3 sources. This will boost up the volume of soft audio samples while leaving loud samples as is. This reduces the dynamic range, but should help for catching quiet audio in noisy listening environments. It takes a floating point number as its value, from 1.0 (off) to 4.0 (blow out your speakers from the background sound of elevator doors closing). 1.5 to 2.5 is a useful range.
You can access drc under the audio tab - on the right hand side you will see DRC - click the icon underneath it and you can change the DRC from off to a value using the slider
I haven't tested it, but would like to know how it works - and what value you ended up using