Are you asking if there is a way to respond to a post via speech? As in, I just talk to my device?
I know Google Pixel has the ability to "type" via speech in the keyboard and works well. I imagine that there is something similar via Microsoft as well, and iPhones. I don't think there is necessarily a need for it to be an extension for Flarum as it is a solution that's built in natively on many devices.