Of the two examples I gave?
The first literally means 'I am unable to hear a sheep'. The second means 'I cannot hear a sheep'.
I guess naturally I would default to saying the opposite of what my friend said. Meaning:
If they said 'I am able to hear a sheep', I would say Chan urrainn dhomh(sa) caora a chluinntinn.
If they said 'I can hear a sheep', I'd say Cha tig agam(sa) caora a chluinntinn.
If my friend said 'I am hearing a sheep', again, I would counter that and use the original example - Chan eil mi(se) a' cluinntinn caora.
I added the emphatic forms of the personal pronouns in brackets because if you were replying to someone in that way (as in directly countering their statement) you would always use the emphatic - they would say mi, you would say mise etc.
Having read your question again, the sentence you actually asked about was 'I hear a sheep'. That in Gaelic is 'Cluinnidh mi caora', so the counter to that would be Cha chluinn mi(se) caora.
It's a slightly different meaning. You're saying that you are unable to hear a sheep, so you're really describing something about yourself (I am unable to hear a sheep). This is saying what you are doing (I am not currently hearing a sheep)
Really it's another weird phrase that English speakers use
Okay here's some dumb for ye.
At first I thought this was about having a hearing dog (dogs that assist deaf people)
Then I realised it said sheep so I thought. "oh this must be a lil joke about having a hearing sheep."
Noticing 'agam' wasn't there I typed "I am not a hearing sheep" because tha mi gòrach and I thought this was all a joke.
What's 'galaxy brain' in Gàidhlig?