No. Redmi 6A does not have Dolby Digital sound support.
Yes, Redmi 6A is a dual sim mobile phone. It also has a dedicated MicroSD card slot.
Yes, Redmi 6A can play Full HD videos without any problem.
No. Redmi 6A does not have the IR remote sensor blaster.
The Redmi 6A is a fairly good update to the Redmi 5A, which itself was barely any different compared to the Redmi 4A. You now get a tall screen, better processor, and improved polish overall.