I don't know about SPDIF passthrough, never used one.
To play AC-3 format in DirectShow you need to install
DTS/AC3 Source (version for 32-bit systems). After the installation of this module Ac-3 streams will appear as movie files without video in BSPlayer, Media Player and other DirectShow frontends.
The best choice for audio playback is of course a dedicated audio player, such as Foobar2000 or Winamp. But getting passthrough working there requires additional steps, or might be completely impossible.