If you haven't already, I think you should speak with the breeder.

Do you know if the breeder has a website?

I ask because I found this contract on line from ONE sphynx breeder. http://www.doggydayscatnapacres.com/sphynxcontract.html It specifically states that no kitten will be sold with even a slight heart murmur. So if nothing else, the breeder will want to KNOW that one kitten in one litter developed such a condition.

The breeder may actually have some suggestions which none of us will; depending upon how much experience the breeder has.

I do think you are right to return to the cat to one of them and get your money back. This cat is currently not only with a heart murmur but also with many symptoms. This should have been disclosed to you before you even put down a deposit!

What a difficult situation.