Hello, I'm TV Tech1 and I'll be working with you to identify and help you resolve your problem.
The first question is, when you did press the mute button on your remote, did you see the speaker icon with the slash show up on the TV screen?
Second question...when you press the TV's volume control, does the slash disappear and you only see the speaker icon or the volume bar appear or does it state "fixed" or "variable" or show speakers off on the screen? That's usually what appears if your TV speakers have been turned off and the TV is delivering it's audio to the Optical or RCA Audio out jacks on the back of the TV set.
Observe what the TV displays first.