Unfortunately, I think all these small hydros are sealed units (I've got one too). No way to change filters, check or add fluid, etc. I guess they assume a garden tractor doesn't see hard enough usage to worry about it, or that the average garden tractor owner isn't smart enough to handle things like fluid checks (dumb assumption /forums/images/graemlins/mad.gif).
Anyway, the procedure for bleeding air out is to first put the transmission in free-wheel mode. On mine there is a metal rod you can pull out and latch, or let it pop back in by spring pressure. It is located at the rear of the tractor by the right wheel. If I remember right, the hole the rod comes through is keyhole shaped and you pull the rod out until a collar comes through the wide part of the hole, and then push it down into the smaller part of the hole to hold it there (this is just holding the internal relieve valve open I think). Once in free-wheel mode, start the tractor up and slowly push the forward and backward pedals to full travel several times. Release the rod to get out of free-wheel mode and try it out.
If bleeding the air out doesn't help, there probably isn't much choice but to take it in and have it looked at. As I said, these are usually sealed units and even checking the fluid would require some major disassembly which might be more than you want to get into.
Good Luck
/forums/images/graemlins/smile.gif
ps. If you work it hard and get the transmission hot (I have many hills), my garden tractor will lose power too. This is just from the fluid thinning when it's hot and slipping too much past internal clearances. This is annoying, but fairly normal I think. All is well again when it cools off. I don't notice any unusual whining when it's doing this, however, so that may not be what you are experiencing.