"Hi-ho" is an exclamation designed to carry some distance because the "h" sound, at high volume, encourages the speaker to empty his/her lungs, putting a great deal of amplitude into the call. You can imagine it being used by one party on the far side of a valley to hail another party on the near side. It only calls attention, and carries no meaning unless one has been agreed to in advance. The dwarves use the call to signal each other, throughout the depths of their mine, that their workday has ended. Since this exclamation is not regular speech, it can take various forms. I don't know how Wagner used it, but the form of the exclamation might have been slightly different than the one Disney used. Given the little of Wagner I've heard, it was likely to have simply been a shout or other exclamation carrying no message other than some profound emotion - either joy or grief.
A "Hoiho" is a yellow-eyed, asocial penguin found in New Zealand.