If you watch the episode 8 promo you can hear what appears to be RJ's voice.
The man saying "the game is over and I won". So I went back a few episodes to listen to every suspect's voice, I even listened to Partridge's, just to make sure.

It turns out the voice on the promo is different from all suspects, which is weird since we know for sure one of them is in fact RJ.
The only two I can 100% say that the voice doesn't match are Stiles and Smith.
Haffner, Bertram,  Partridge and Mccalister all have different voices when compared to the one on the promo, but not too different that I can clearly rule out.
Based on voice alone Thomas was the one who came closer, if you disregard the subtle accent.