Hello, I'm TV Tech1 and I'll be working with you to identify and help you resolve your problem.
There's a tiny mechanical switch on the SD card reader that's probably jammed. Really minute switch. You may actually have to disconnect the SD card reader module from the TV's main board to get it to restore normal audio. If it's a part of the main board, you can't get the SD card reader as a separated module.
One thing I would try is repeatedly clicking /unclicking the SD card maybe about 20 times. The problem is that we rarely use the SD card readers on TV sets and the switch just jams up inside the TV. And I'm not really too keen on attempting to lubricate the SD card reader to unjam the switch. Graphite's conductivity scares me...but WD-40...I'm afraid a big shot of that in the SD card reader could get between the glass and the plasma panel on some models...can't recommend that.
So. Try installing/uninstalling...get that switch moving. See if we are fortunate or not.
Lastly...I'm not too keen on this but it may work...Unplug the TV set for 15 minutes to reset the computer in the TV set. I'm leary we may end up with the TV set never starting up because of that bad switch, but it's a chance we have to take to see if the micro is actually misbehaving due to the SD card playing over and over. I don't know if you had it doing that or not, but I want to cover all possibilities so you can inform me more about your situation.
So...bases covered. Let me know which ones apply to your situation.
If your TV is younger than 2008...don't spray WD-40 in the slot. Better to take the back cover off and have better control over the spray.