I’ve assumed Ackbar is giving the mark to the capital ships, with Lando giving the mark to the fighters.
Admiral Ackbar says “prepare to jump to hyperspace on my mark”, and Lando answers “all right, stand by”. And then he jumps into hyperspace!
He says, “All craft, prepare to jump into hyperspace on my mark”.
There is a trilling beeping noise right before Lando hits the lever. I always assumed that was the “mark” broadcast to all ships.
I only hear the engine revvin up