I thought you were looking for something to even out the volume...maybe I misunderstood.
An audio compressor has a threshold, a ratio, an attack time, and a release time (and sometimes a knee). Basically, once the audio level passes the threshold, the level is only allowed to raise based on the ratio. Lets say you have the threshold set at -20db, and the ratio set to 1:4. Once the audio level crosses the threshold (-20db), then the input volume has to increase by 4db to get an output increase of 1db...IE: if the input level is -16db, then the output is -19db. A 1:5 ration means that every 5 db increase raises the output by 1db.
What this does is smooth out the volume level. It's very easy to overdo compression though, and get rid of all dynamics, so be careful!
I assume that at some point in the signal chain (I assume at the 'Output Format' stage), the 5.1 audio stream is converted to stereo, so that is why I said to put the compressor after that. You can also use a multi channel compressor to compress all 6 streams (5.1 surround), but I haven't found a good free option for that yet. I have played around with the multi channel Deft compressor, and it appears that it will work good, but it's $70.
mm0