Wahoo indeed. Seemed like a long week. Just have to get the RealTek onboard sound going again and I’ll be cooking with gas. Must be clicking in the wrong area. That’s the problem with fixing issues every 3 or 4 years - you have to relearn everything all over again.
EDIT: Well got sound by plugging in directly at the MB not on the top of the case like I used too. Oh well.
The issue is that your "front panel" sound (what you used before, on the top) is not configured properly. The RealTek interface changes from version to version, so it may look different but will essentially be the same. First, make sure you have the speakers configured properly. Then use the configuration/option settings (usually looks like a gear icon) to set the Playback the way you want it. (If this isn't set up as it was, the behavior might not be like it was before - although I'm not sure why changing the board would cause this, since your drive technically didn't change. Maybe the RealTek software recognizes individual chips/boards???).
I'm attaching a picture, but I don't know how it will look since posting pics has become problematic...we'll see. If I'm following you, you want the rear panel muted when a front panel device is plugged in - and you *do not* want the front and rear output devices to play two different streams simultaneously. The reason there's a choice here is so that you could have two different audio sources (think in-game comms like Discord or TeamSpeak); this allows for voice chat through a headset, while still having the game sound come from your speakers.