I have a 2013 ES and kept saying to my wife "Wish they had a mute button for the radio". There are times when you don't want to turn down the radio but briefly just want to mute it. Yesterday, I was reaching over to open the vent on the passenger side in the middle. I accidentally hit the button to the left of the tuning dial and the radio sound went off. I hit this button again and sound. It is a small button with an arrow pointing to the right. Now I am sure this is in the manual but who reads Lexus's version of War & Peace.

 

You can also hold the MODE button on the steering wheel for a few seconds and it will mute the radio/media.

 

Yep, I found it.