short answer: download ac3filter, install ac3filter, problem probably solved.

Longer answer: if you don't want to use ac3filter, ffdshow indeed supports ac3 playback. However, you might need to go in and tweak the audio decoder settings. If you're using CCCP (which I'd recommend for most people to begin with), you shouldn't be having problems at all anyway, but you might want to reset codec settings. If you're using some other codec pack, or individually managing codecs and playback filters, I'd strongly urge you to ditch that route and go CCCP unless you've got good reason not to. It'd make everyone's life easier, including your own.