Am I responsible for, and can you make an appeal to Medicare after 9 month of receiving a denied bill from an ambulance service?

Medicare has established a given time within which to appeal; that period begins running either from the date of the Medicare benefits notice or mailing date - I don't recall which now.

How do you know that the ambulance service received a denial within 2 weeks? That would be an unusually quick time for Medicare claim processing, unless the denial occurred close to the time Medicare sent out the benefit notice.

What did the Medicare notice indicate about payment? If you received an EOB with a nonpayment notation, the appeal period would run from that Medicare notice.

This is what I found regarding Medicare and ambulance service. https://www.medicare.gov/coverage/ambulance-services.html