もう一回言ってくれませんかって英語でなんて言うの？

In most occasions, if you didn't hear or understand exactly what the person said, you can immediately say, "I'm sorry", while gesturing towards your ear to indicate you didn't catch what they said. Or, you can say, "I'm sorry I didn't catch that". "I didn't catch that, I'm sorry".

When you don't hear someone correctly the first time, then you can ask them to repeat themselves in the following ways:
-I'm sorry, could you please say that again?
-Pardon? I didn't catch what you said the first time.
-Pardon me; could you please repeat that?

If you are trying to say something in a polite way then you would start the sentence with 'I'm sorry'
So the best thing to say would be 'I'm sorry could you repeat that again please?' or 'I'm sorry could you please repeat that?'

Sorry, I don't understand. Would you mind repeating that again? - This expression can be used to request the speaker to speak the same sentence(s) you didn't understand.

Could you please say that again for me? I didn't catch that the first time. - This expression can be used to request the speaker to speak the same sentence(s) you didn't understand. In this sentence, the word "catch" means "understand".

"Excuse me, could you please repeat that?" is something you would ask someone if you did not hear what they said correctly or if you didn't understand what they were saying.

The two sentences you see provided above are great ways to express to your listener that you could not hear them and you want them to repeat themselves. In the first example you will notice the term make out. Although this term has different meanings, in this sentence it means to understand or grasp. The first sentence is appropriate for informal settings while the second sentence is appropriate for formal settings.

When you can't hear/catch/understand what someone says, you want to ask them to say it again - in a polite way. Above are a couple of suggestions which may be suitable for this scenario.

I didn't catch that, could you say that again?'
'Catch' in this instance means to hear or understand. By saying you didn't catch it, you are saying you didn't understand and would like them to say it again.

'Sorry I couldn't quite hear that, could you repeat it?'
To 'repeat' something means to do it over or say it again.

'Pardon?'
This word is used in The UK and is a polite way of saying 'what?'. It implies you did not hear and that the person you are talking to needs to repeat themselves.

Your voice was very faint - please repeat what you said.
If someone's voice is faint, it means that they spoke too
softly for you to understand what they were saying.

I missed that - please repeat what you said.
This expression is used when you did not understand
or did not hear what the speaker just said. They will
automatically repeat what they said earlier.

When you couldn't hear/understand someone, and want to ask them to repeat themselves, you can say:
"I'm sorry, could you please repeat that?"
"I didn't quite catch that, can you say that again?"
"Come again?"

The first two examples are very polite, but the 3rd is informal for a friend.

American culture is more direct and casual, and the British is more polite.

Its a formal and polite way to show that you did not hear what was said and you would like then to repeat.