Q: Quicktime 7 Pro can no longer mux Ac 3 surround files with .mov files (OS 10.8.5)

I just made a Dolby 5.1 Ac-3 file (Auto settings) in Compressor 3.05. Quicktime 7.6.6 Pro will not open it to paste into a h.264 file and combine (mux). I tried a known good Ac-3 file from a 10yr old Protools session, also without success. This used to just work for me! Its all my own content - no piracy here!

I've read a lot of contradictory information about this related to recent OS updates, but I thought it would still work in 10.8.5. I also cannot open the AC-3 file in Quicktime 10.2. Anybody with recent experience of resolving this?