Yes, you can use 2 4G SIM cards at the same time in Nokia 8.1.
There is no official communication from the company if Nokia 8.1 will be able to work on the 5G network or not.
Yes, with Nokia 8.1 you can record 4K videos @ 30 fps.
Yes, Nokia 8.1 is a dual sim mobile phone.
Nokia 8.1 has a single speaker with smart amplifier.