Something was just playing on my Mac. Probably in Safari. I have tabs open in Safari and Chrome. IIRC a sound icon used to show I the URL area if something was playing in any tab and you could click on the icon and stop whatever was playing.
How does one find out what is playing (probably a video in Safari) on one’s computer or more importantly just stopping it? Can use the hammer—mute (which I did I this case).
I just unmuted and now nothing, so assume the audio got to the end. The added complication is that I define the function keys with Keyboard Maestro. At least on my MBP I can use the fn key to remap as Apple intended, but no fn key on my iMac.
In the address bar, to the right, should be a blue speaker/sound symbol. Click on it and it will show you the playing tab(s). You can mute them from there too.
You should be able to disable auto-playing videos in Safari (and maybe Chrome). Don’t have my iMac in front of me so can’t check at present but I am pretty I did this many macOS’s ago.