If it's AC3 sound you can use "Gains"and/or "Equalizer" possibilities of AC3 filter ([Start] > Programs > AC3filter > AC3Filter config).
Don't use ffdshow myself but installed it on a slower machine the other day and though I don't know for sure, I thought is also offers a possibility to increase volume?
Otherwise there is the "standard" possibility of the "dedynamic filter" and/or using a WinAmp2 DSP plugin, such as e.g. Enhancer 0.17.