How do I reply to "I'm Sorry?"

I've seen the usage of "I'm sorry", but now I saw those with "?" mark. Been googling it for a while and one of the results said it's the same as "Excuse me?", asking for confirmation. So how do I reply to that?

I can only answer on the basis of the spoken word so cannot give references as is usually done on this site.

The pseudo-question “I’m sorry?” is used in the sense of “I heard what you said but do you really mean it?”, “What? Do you expect me to believe that?”, “You’ve interrupted me. What do you want?”, or “I don’t agree with you. Explain yourself”.

It is often often delivered slightly disdainfully, as if the speaker can hardly believe there is any possible explanation for the statement or interruption.

The best reply is to act neutrally and politely and to answer on the basis of one of the above understandings.

Edit: I am grateful for comments that add the idea that Excuse me? may be simply a request to repeat what was not heard.