I was sitting in my garden last week on a quiet afternoon and realised I could hear an aircraft. I looked around and could see nothing near me, despite several routes almost overhead at low level arrriving into MAN which is about 11 miles away. So I looked further into the sky and there were two contrails, clearly at cruise level and they were the only thing near (confirmed by a quick look at FR24). So I assume that aircraft a 35,000ft can be heard easily under the right conditions.
Remember 35,000ft is about seven miles and I can hear aircraft taking off from MAN if the ambient sound is low.