mute button does not unmute Sonos Beam

I've seen previous articles about Sonos volume control - that's all working fine for me.  vol+/- as well as the fixed volume steps (10%, 30%, etc.).  However, the mute button sets the Beam to mute, but cannot toggle to unmute as well.  I tried configuring a long press on the mute button to trigger unmute command, but no luck.  Any suggestions?