It's likely that WB already has the rights to his likeness as the Terminator contractually, so no extra money is required. If they want a performance, they'd have to spend money (probably a lot of money).
So our options are:
- The Terminator is silent.
- Arnold impersonator
- Speaks with a glitched robotic voice.
If I had to guess, I go with that order in terms of likelihood. Not because I want a silent Terminator, but I could see a possibility of contractual issues between Arnold and WB over option 2).