Ah ok, yeah a kidney thing is a bit more serious than mine, but if you've had a look through the CASA stuff, you should be ok. It's been a while since I had a look at that stuff, and when I did, I was scanning it with an eye out for things affecting me :P
With regards to the hearing, they listed specific levels of hearing loss that were the cutoffs, and I was close on some (having had hearing problems since I was a child) so I had a hearing test which showed I would pass the CASA requirements. I actually did that before I applied even since I didn't want to get disappointed if I was ineligible from the start! That only cost me $60 at an audiologist, but unless you know you have a diagnosed hearing problem already, you'd probably pass easy, the limits aren't too restrictive. I don't know where you live, but there is a place we had in QLD called Bay Audio that had a basic hearing test you could do for free which would tell you if you needed a further test. If you passed something like that, you'd definitely be fine.
By the time they ask you to go for a medical, they've offered you a job (well for me it was at least). Then they organise your visits to the DAME and an ophthalmologist, and the drug test. Given they pay for all that, I guess they don't want to bother on people they don't want :P