1. 좀 더 크게 말씀해 주시겠습니까?

Could you speak louder?  ------>  Would[Could] you speak a little louder, please?

2. 크게 말씀해 주시겠습니까?

Could you speak loudly, please?  ------>​  Would[Could] you speak up, please?

3. 좀 작게 말해 주세요.

Please speak quietly.  ------>​  Can you speak a little softer?