Do I have to hear a sound or smell something to be sure that I have to repeat my ablution? What about people who suffer from excess gas and hear sounds that make them doubt?

May Allah reward you well.

In the name of Allah, all praise is for Allah, and may peace and blessing be upon the Messenger of Allah and his family. To proceed:

Whoever passes gas has nullified his ablution. He does not have to hear a sound or smell anything as long as he is sure. The hadith referred to applies to a situation in which there is doubt. As for someone who is sure that something has exited his body, it is not a condition that he has to hear or smell anything; and Allah Almighty is more exalted and more knowledgeable.