The robots voices in Mann vs Machine are simply the standard, human class dialogue passed through a robotic filter.

Among these voice files are unused Demoman and Soldier lines, generally stating "DOMINATED!", followed by expressions of concern or friendship.

It's likely that the original, human versions of these lines were created for the WAR! Update, in which the BLU Soldier and RED Demoman were discovered to have a secret friendship, and the administrator pitted them against each other. The unused voice clips imply that the Soldier and Demoman were putting on an act for their bosses, pretending to no longer be friends. It's unknown why Valve chose not to include these lines.
Contributed by MidwayCJ