Unless you want to write custom software for it that interfaces with the volume mixer to check which applications are actively playing audio, the most feasible approach may be to consider which applications would be playing audio in the scenario you're wanting to apply this concept (E.G. a game, a media player, a browser), and reduce the volume for those specific applications using a number of "Set Audio Level" actions (with a check to see if the applications are actually running, if the log message are an issue), then restore the volume once you're done playing back TTS audio.