Sorry.

I'm pretty sure its him. It sounds like more than one person screaming which would make sense because he was made out of alot of souls.

If you listen carefully, you hear a woman say "All right! Yeah!" The noises are coming from the prisoners in the level for certain, but maybe from Ermac also.
A couple other observations in the IGN video. Cyrax actually holds Ermac against the prison bars, which is cool, because it means the prisoners have an effect in the outcome of a match.
Also, NEW RULE: OLD PEOPLE LIKE SHUJINKO ARE NO LONGER ALLOWED TO SAY "Get over here!" This is the equivalent of an 80 year old woman saying "Where you at?!" in that one phone service commercial.

The multiple screams are coming from Ermac. You can hear them when he gets knocked off and right when he lands. You can also slightly hear them when the harpoon hits him.
They are all on time when Ermac gets hit, so they are his(really their voices).

Yeah its really cool.

if you watch one of the videos on gamespot where sub zero and ermac face off subby sends ermac off the ledge and he screams like that the whole way down...

Ermac makes the multiple synched scream when he's falling to the lower level, AND the prisoners are also screaming, and yelling stuff. A silent prison would not be right.
Ka-Tra