Lots of things in the world are possible! It is however, currently against policy to play audio in an extension (see here). Another relevant policy point:
Extensions may not transact or encourage the transacting of monetary exchange in relation to any non-Twitch/Amazon commerce instruments.
The very first thing that pops into my mind is whether you’d want a message like this baked into the video stream so that it is heard by everyone (and is available in the vod). If this is the case, I would be looking at a very different solution.
The tl;dr: extensions may not currently be the technology best suited to achieve your goal, as stated above.