Under the video option (very bottom option), there is a setting for Audio. I changed that to ATI HDMI Output, since that's how my receiver is connected. it had previously been set as "same as audio" but this didn't work for me, even thought the audio selection was set to ATI HDMI Output.
Also, under the DSP options (different area - not in the regular options section), there is an option for converting or transcoding (can't remember, I'm at work now), and I changed everything there to same as source or similar.
these 2 changes seem to have gotten all of my video issues resolved.
it's possible that you have the file type options for mp4 wrong, but I would try the things above first.
I'm ABSOLUTELY not an expert in any of this, I've just spent the last several days messing with all of it trying to get my setup working.
I have most of the video file types set to use LAV Splitter, ffdshow video, LAV audio and madVR, and this gets everything playing with sound.
I hope this helps