I would like to send a vocal message with my microphone when i donate.
Is it possible ?
I dont think so…
So why ? 
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.
Because the Video-On-Demand doesn’t show what was done through extensions, correct?
Nevertheless, it would still be the most easy-to-setup, widely available solution for both, the streamers and viewers, I can think of.
I’m beginning to work on a similar project, where users can queue and the streamer allows for a short audio conversation to occur on a one-by-one basis. Similar to what happens on radio stations when calls are received. Prototype -RSC- Radio Show Call.
I will open a new thread to request for volunteers, but thought about writing here too since I found the concepts similar.