What is selected output of AC3 filter at box "Output / Speakers:"?
[Start] > Program files > AC3filter > AC3Filter config (> [Main])
(Bottom choice of the list is SPDIF)
Also check what is selected under "Speakers" on AC3 filter tab [MIXER].
(No luck? Check "Default audio renderer" at (southeast quarter of) [System]-tab of AC3Filter's properties and perhaps also check BSPlayer's settings under (rightclick >) Options > Preferences > Audio.)