I've not done a lot with co2 stuff but they type used also depends on the location in the gun and what the seal does.
I know co2 seals can freeze which is totally opposite to say a pcp inlet orings where it can get overheated /crack a lot quicker that any other orings in the gun.
It also depends on if its a static orings or a dynamic one.the latter is better in viton as they wear a lot less than std nbr stuff.
Cracking sounds heard can actually just be grease/lubricant on the seals/internals moving when the gun is pressured up in both co2 and pcp.
Poly seals can be hard wearing but can only be used in certain places.
Not sure if this has helped but there you go