Can you combine the Ring and the Revolver to attack? One of the heroes was attacking with a line of direct sight with the Revolver and was using the +1 die bonus from the Revolver. They also had the Ring and claimed that they could use the Ring to roll for Sanity instead of Speed while still using the +1 bonus from the Revolver and attacking with a direct line of sight from the Revolver. Is this correct? Or can one only use the Ring or the Revolver, not both?
No. You can use the Ring or the Revolver, but not both.
The Ring reads:
If you attack an opponent that has a Sanity trait, you can attack with Sanity instead of Might.
The Revolver reads:
You can use the Revolver to attack with Speed instead of Might. [...]
If you use the Ring, you are attacking with Sanity (not Might), so you can't use the Revolver.
If you use the Revolver, you are attacking with Speed (not Might), so you can't use the Ring.
In practical terms, one can attack an opponent's mind using the Ring, or one can shoot an opponent's brains out using the Revolver, but one can't attack someone's mind with a bullet, and one can't blow someone's brains out without something physical.
They [...] claimed that they could use the Ring to roll for Sanity instead of Speed while still using the +1 bonus from the Revolver
That's outright incorrect. The Revolver's bonus dice applies specifically to Speed attacks.
Roll 1 additional die on your Speed attack roll