UPDATE-
I submitted the issue and it seems that for now there is no solution to the problem.
It seems that the mobile browsers are the problem and the C3 team cannot control it.
Alas, I noticed that I was able to use speech synthesis on other webapps with my phone, so maybe there are ways to bypass the problem or to create a different C3 plugin for mobile speech synthesis.
It is way far for my scope, but I'll be glad to know if there will be a solution in the future.