My father has severe hearing impairment and both my parents are in constant stress as a result. My mother has been fighting cancer as well. How can they cope?

You may want to call your local adult social services. They can do a welfare check and determine what should be done. Your parents' health is endangered by all of this stress, as you note.
You can also see if you have an Area Agency on Aging in your community (www.n4a.org). They may be able to help you.
We'd love to hear how it goes.
Take care,
Carol