[Jan-Poo]

Are you sure you want to know? It should be revealed around the end of the anime. I think you should wait until then. Anyway since you asked I'll write the answer here. Then the choice to read it now or later is up to you.

Spoiler for Novel Episode4 near the end:

Maria in the past was singing a song to Rosa, but since she didn't remember all the lyrics she started humming the parts she didn't know. That's how the "uu-uu" sound was born. Since Rosa happily listened to Maria's singing, that was a very dear memory for her.

So "uu-uu" became a magic sound Maria used to make the bond with her mother stronger. By repeating "uu-uu" Maria hopes to make Rosa happy like in that day, it is a magic that should work to make them close and happy.

That's why whenever Rosa gets angry, Maria answers by saying "uu-uu" even with more strength than before. Sadly this only causes Rosa to become even angrier.

Despite this fact, Maria says "uu-uu" in many other situations, even when Rosa is not around. So it can be assumed that Maria's "uu-uu" became a compulsory behavior.