So, if you look at a joint like 12U, that's just not soldered. I can see the entirety of the black wire going through that upper hole, and there doesn't look to be any solder on it. This same issue is present where the UF4007s mount, the ground tab on the black RCA jack, 3L,
A4/A5 looks like a place where you need to remove a bunch of solder.
I would recommend obtaining an 1/8" TRS to RCA adapter cable so you can use your phone as a source. It's difficult to debug both a new build and a potential system issue all at the same time.