It sounds like a board or bezel button issue on your set. Try tapping the bezel where the volume buttons are to see if that helps. It may be "stuck".
If that still doesn't help, it may indicate an issue with an internal board which might require service. For that, or further troubleshooting, try calling the Samsung customer service center at 800-SAMSUNG.
I have a Samsung UN55C6400RF LED television and suddenly the volume goes to 100 percent and won't come down. I unplugged the unit and it still stays on 100. I took out all the batteries of all my remotes thinking there might be a "hot" remote. I can briefly use the remote to turn the volume down but it immediately goes back up. The red LED on the front panel continuously flickers which makes it look like it's receiving an IR signal. Also, none of the touch buttons on the front work.
Any ideas on how to fix this would be appreciated.