Check if setting of Output device under "Options"/"Preferences"/"Audio" is e.g. "default direct sound device". (Also that "For AC34 decoding use <AC3Filter> is selected?")
Further check if the different selections/settings (on the various tabs) of AC3-filter correspond to your soundsystem. ([Start] > Programs > AC3filter > AC3filter Config.)
(While playing a movie you can also reach filters by right-clicking on the movie after which select "Options"/"Filters"/"Advanced")