It depends on your expression. Expression Examples: mad, sad, happy, angry, clueless and etc.

Sorry I didn't understand you, can you repeat that slowly for me, please? Thank you! Sorry, I didn't understand you: could you pronounce your sentence phonetically, please? Or Sorry, I didn't understand you, can you speak slower please so I can understand you easier? Thank you! or Sorry, can you repeat that please? Use manners because you're asking someone to say something again and they might be annoyed if you don't use manners.