For the question that you don't know the answer or you answer isn't on your field, maybe you should say: - To be perfectly honest, I afraid that I don't have an answer for you. - I'm don't have much experience with this type of question... - I'm not sure what you mentioned, because I don't have much experience in that area. Then If you want more time to think and answer the question latter, you should say: - But get me get back to you with an answer by tomorrow, Is that alright? - Why don't you leave me your contact info, and I'll have an answer for you by tomorrow. .... For friends, you might just say directly: I don't know what the answers is...

At first you should say: Pardon me or excuse me. I'm sorry, I didn't quite make that. Then they will repeat the question, If you still don't get it, you would say: Could you say that again? or Could you please repeat your question again? (more polite) If you still don't know what they mean, you should ask for the third time, now you say: I'm really sorry, but I'm having a problem understanding what you're saying. I don't know why, but can you please express it in another way? .... Actually, when I talk with my friend, I usually say: Hey, what? sorry, I'm deaf, what have you said? :D