This mod is now included in DBM (and has been for some time). However, it still works - if all you want is a warning.
Super simple. No config, no slash commands, no windows. Addon plays the NPCScan style warning alert when you are next in line to Brawl. Plays on the master channel, so even if you have your sounds disabled in the game, you should hear it. Make sure to turn on background sounds if you want it to notify you when you are tabbed out.