I just went through a lot of pain trying to get passthrough of some m4v files. Everything else worked ok, but these didn't. I found graphedit http://en.wikipedia.org/wiki/GraphEdit really useful to see what was going on. In my case the splitter used was not what I thought (MPC-mp4) and it sent the output back to microsoft filters independent of what I had set in MC (i.e. AC3). The Microsoft filters only selected the stereo stream. The other splitter (Haali I think), showed that it could get the AC3 stream but the video wouldn't play. Ended up just remuxing into mkv files and everything works a lot better.
Anyways, try graphedit.
Craig
I dumped the graph info and a part looked like this:
Filter 'Haali Media Splitter'
CLSID: {55DA30FC-F16B-49FC-BAA5-AE59FC65F82D}
Host: C:\Program Files\Haali\MatroskaSplitter\splitter.ax
Output Pin 'Video'
Connected to pin 'In' of filter 'ffdshow Video Decoder'
Major type MEDIATYPE_Video Sub type Unknown GUID Name: {31435641-0000-0010-8000-00AA00389B71}, Format type FORMAT_MPEG2_VIDEO
Output Pin 'Audio'
Connected to pin 'Input' of filter 'ACM Wrapper'
Major type MEDIATYPE_Audio Sub type Unknown GUID Name: {00002000-0000-0010-8000-00AA00389B71}, Format type FORMAT_WaveFormatEx
It shows that even if I asked for ffdshow Audio i get ACM Wraper as audio decoder... Do not know what it is or how to get rid of it.
A thing that can be a bit confusing in the AC3Filter configuration is that even though you tick the boxes for AC3 and DTS under SPDIF passthrough in the SPDIF pane, it won't use SPDIF until you tick the box next to Use SPDIF under Output format on the left. Also select AS IS (no change) to be sure it doesn't rearrange the channels in any way.
When forcing AC3Filter, the AC3 Filter is used but I am not able to get anything else through it than 2ch stereo
I can try to remove all filters and decoders and install new by using the CCCP collection. I hesitate to do it because it is a lot of work to set up everything all over again...
I am out of suggestions......