I would be a bit paranoid too. Sucks man. Given the situation, you probably want to address the common OT issues as though it could be a software bug although probably unlikely. Load a new project just in case. If you have another memory card try it. If you don’t have another card, try powering on without the memory card inserted and check for improvement. Run the error test in the startup menu just in case, see if anything pops up.
Since OT has headphone volume independent of the outputs, and you checked for stereo output, we could say that the primary audio is unimpacted. I forget, aside from the main headphone volume and the stereo L/R pan, does OT have split panning or split audio for the headphones so you can hear the cue from A and B in L/R ears independently? If it does, be sure to check that.
I know you didn’t do anything specific, but check it anyways.
Can’t help you on cost estimates. Will be easier to determine if something can be ascertained from your tests, but the most likely scenario is that it has to go to elektron or a qualified tech to check it out. I usually suspect the main analog volume pot itself (like the headphone volume pot specifically b/c it’s split L/R so sometimes one side of a pot fails) but with no crackling, there’s nothing to point to a hardware issue yet so I wouldn’t jump to any conclusions.
Do a little more digging and see what kind of data you can come up with. Probably makes you mad or depressed but try and approach it analytically. Sometimes just a correct diagnosis, even if not an immediate fix, can still make you feel a bit more in control of the situation so come at it from that angle.
Another thing you can do if you’re comfortable opening it up and none of this other stuff yields anything, is with MD and OT specifically, there seems to be the possibility for some kind of association between the ribbon cable connections on the PCBs and weird issues, so sometimes people will unplug the ribbon cables and reseat them between the PCBs. I wouldn’t get out your hex driver until you try the other stuff though.
Another thing I’ll say, only because you didn’t mention it specifically, is if your headphones use an adapter to convert 1/8" to 1/4", try a different adapter just in case.