Ok, admittedly I haven't had much experience in troubleshooting audio issues in the past, but I really can't wrap my head around this one regardless.
I recently replaced my usual desktop speakers with an unused Denon ADV-500D 5.1 system from the living room. While I switch my PC and PS3 over on it through Optical, I'd much rather have my PC be permanently connected through Optical and have my PS3/WiiU connected to the system's RCA/Phono ports - but it just doesn't seem to want to play; in a number of ways.
Basically, whenever I connect anything to the system's Video 1 or Video 2 RCA/Phono ports, the sound coming back through is incredibly quiet even when you crank the dial up to the highest point. Nothing like the output you can get when using optical and easily to the point where being in such a tiny room as I am means it's completely useless for playing a game with. Though the bass coming from the sub seems unaffected and will still rattle the house - drowning out the sound even more.
What's more confusing is using those Video 1 or Video 2 ports makes the sound come from the "TV" channel on the receiver rather than the Video 1 or Video 2 channel labelled on the back (which is what the Optical port outputs to). But I don't know if that's normal - or even possible to change.
The other available RCA/Phono ports don't seem to output to any channel and no matter how much I tweak the receiver settings through connecting it to a TV or the settings available on its own LCD display affects it in any reputable way.
Basically it just seems like something has gone very wrong with the RCA/Phono jacks. But again, I really don't know. I'm very lost!
I love these speakers, but without the variable connectivity, I'll have to start looking into replacements.
I'm also up for any soldering if that becomes a viable topic.
I'm sorry to say I probably can't be of much help to you. This is the sort of problem which typically requires someone be there, making changes as they listen. When "X" occurs, you then know "2" is the answer. On a forum, diagnosis becomes far more complicated.
On the face of it, it sounds as though there may be a switch set to the wrong position. However, without looking at the unit and how it can switch signal paths, that's strictly a guess. Does the unit have any video copying facilities? If so, then I would make a first assumption the incoming signal is being directed to another input. Check those facilities if they exist and get back to us with the results.
Modern / newer stuff with ROHS compliant solder IS prone to needing a 'resolder' far more than older stuff with softer, lead / tin, compounds used for that purpose. I've had to have 2 pieces fixed in that manner over the years. Both times, NO electronic faults were found but rather just some 'touch up' to what had turned into cold solder joints.
Good luck finding them, if they exist!
Follow Jan's advice before cracking the case and poking around.
DON'T forget. Even a unit that has been unplugged for a while (sometimes hours) can still BITE. And painfully, at that. Caps can hold substantial charge for quite a while. BE CAREFUL.