When Elsie went investigating the signal in the old theatre, she also found that "Arnold" was sending messages to the early gen hosts.

Plot-wise, it made sense that it was Ford preproduction on his final narrative.
However... when his child self host kills puts the greyhound down for killing a rabbit, he reveals that "Arnold" told him to.

This was new information for Ford. So he couldn't have been responsible.

So who was?

The only thing I can think of is that Bernard, modelled after Arnold was responsible for the Arnold broadcasts, the same way he gave Dolores the game off the maze.

Ford made Bernard to help make the hosts more real, with a full range of emotions, because his human technicians couldn't.

Did he also give an AI the task of pushing hosts towards sentience/sapience/whatever?

Or did I miss or forget where the voice of Arnold was revealed?