Mohammed's and Jesus's claims that they were God/Allah or heard God/Allah, are unsubstantiated.
Someone claimed to have read an Islamic word in a Kiwi fruit once, years ago. When you look at the clouds you see shapes, it's just imagination.
One test would be to get two people who hear God, keep them seperate, and see if they corroborate each other's claims. You would have to be VERY careful to check for hearing devices, or transmitters.
The rooms could be shielded for conventional radio signals. If they did corroborate, (A MIRACLE), then I would look for someother means as to how they communicated.