I just had the exact same problem with Audacity 2.4.2 in OSX 10.11 as someone else in Windows - suddenly playing without sound and nothing would stop it. All menus were greyed out. Play and stop buttons were highlighted, so it looks like both were stuck on. I don’t use the buttons, so I suspect I butterfingered some shortcut key combination that set that off.
I had to wait for it to play to the end of the track. Only other option was to tell OSX to force quit it. All is back to normal now. Play/stop works via spacebar or button.