Here's the answer:
“もういちどおねがいします”
(mō ichido onegaishimasu)
Watch a real native speaker say it:
Here's how it sounds in a textbook:
You could also say:
- もういちどお願いします
- もう一度お願いします
Fun fact: the hundreds of thousands of people learning Japanese with Memrise get this phrase correct 84.76% of the time!