No bright ideas here, but I glanced through the Yamaha manual (a very exciting coffee break). I'm sure you've already tried everything in the manual, but I'll throw out a couple of simple (minimal ridicule, please) suggestions from the book.
p32 - To enjoy audio from the MULTI CH INPUT jacks with a video source, first select the video source, THEN press Multi Ch Input.
p36 - Input mode can be AUTO, DTS, ANALOG. When using AUTO, the receiver looks for digital then analog signals. Maybe try ANALOG?
That's it...pretty weak, I know, and it doesn't explain why you get sound for 15 secs that fades away?