We've run into this a lot. We're all tabby cathy's and we tab out and have to message each other to remind each other what we were doing 2 minutes ago. Is there anyway it can play when the tab isn't in focus?
It's actually not a matter of if the tab is out of focus, it's a matter of if you have interacted with the page yet or not (i.e. clicked it).
Chrome, Firefox, and other browsers disable "autoplaying" any audio until a user clicks the page. So it's best to either click the page, or you can try and enable "auto-playing" on PopFlash.
On Chrome: click the lock in the address bar and hit "site settings", then select "Sound" and set it to "Allow".
On Firefox: https://i.imgur.com/XKn2Kyj.png click the "play button" with an x through it after the PopFlash accept music starts, and select "Allow Audio and Video".
This is probably confusing, I'll try to write up a guide. Chrome gives YouTube this ability out of the box, but sites like PopFlash don't get that luxury.