Here's the answer:
“kan du snakke saktere, vær så snill?”
Watch a real native speaker say it:
Here's how it sounds in a textbook:
You could also say:
- kan du snakke saktere, takk?
- kan du snakke saktere, tusen takk?
Fun fact: the hundreds of thousands of people learning Norwegian with Memrise get this phrase correct 97.32% of the time!