The Japanese language has a lot of dialects and indigenous words depending on the region, and some Japanese people do speak fast and use technical terms in conversations. There are a lot of shy Japanese people as well, and they might mumble.
So, here are sentences that native Japanese actually use when they cannot understand what speakers are saying and want them to repeat the sentences.
I am a native Japanese, and for me the Japanese language sounds slow and clear. However, I hear it is not the case for non-native Japanese speakers. So, hope this article would be a help in the conversations in Japanese.
Ask to speak slowly
Ask to repeat the words/sentences
Ask to speak louder
This might sound sentence fragment and cold. However, it is what native Japanese say in conversations with close friends. To talk with sentence fragments and in a very casual way is only allowed within the intimate circles, and it is sometimes how the Japanese show intimacy and friendship. However, if you used the talking style in business, it would be considered very rude and untrustworthy even if two people are very close.