How to ask to repeat
If you didn't understand or hear what the person you are talking to said, and you want him or her to say it once again, what do you usually say? "Pardon?"
Here in my part of the English-speaking world (SE Pennsylvania) "I'm sorry" is just as good as, if not better (more likely to be said by a native speaker) than "pardon". And because not hearing and not understanding are quite different problems, it would be best to say the whole thing: "I'm sorry I didn't understand." or "I'm sorry, I didn't hear (clearly) what you said."
Followed by: "Could you please say it again for me?"