Inside the K-Lite Codec Pack, there's a tool called 'fixcodec.exe'. One of its functions is to boost ac3 volume.
Try enabling DRC or turning the volume up. To actually make it louder though you would need to re-encode. This fixcodec probably just enables DRC and boost in AC3Filter.
If you use the AC3 filter then go into control panel, switch to classic view and you will see the icon for it. Open it up and move the slider for the master volume up a bit. You'll have to experiment with it until you are happy that you have set it at the right volume.