Hi
How many audio tracks are you using? I can't think of an option that will mute ALL the audio tracks at the same time and grey them out with a single button press. Is the loudspeaker icon at the left end of the track/tracks (or any of the individual clip sound tracks red? If it/they are then you will need to click each one again to remove the mute effect for that track/clip.
Have you made (enabled) one of the tracks solo by clicking the icon at the left that looks like a group of pawns. This will temporarily mute all the other tracks and grey them out. If you've inadvertantly done this you need to re-click the button.
Remember you can always step backwards using the back arrow provided you haven't made a lot of editing changes following your mistake.
Nat