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?
Audio: Marantz AV7702mkII / Marantz MM8077 / Emotiva UPA-7 for 7.2.4 Atmos/DTS:X - Video: Sony VPL-VW600ES & SI Black Diamond 133" 2.35:1 CIH for 4K UHD - Sources: Oppo BDP-103D, Mede8er 600d with Synology ds716 NAS (28tb), PS4, DirecTV - 7. Paradigm Studio 20 v5 (L, R, FW), Studio CC-590 v5 (C), Sonance VP63 (SS); 2. DIYSG Dayton HO 18" (4); PSA S3000i (3); 4! Infinity Entra One (FH), Sonance VP65R (TR)
Lewis Family Cinema 2.0, Lewis Family Cinema 1.0