Is it "τις" because of Accusative?
Yes! Here's how definite articles change in accusative:
το -> το
οι (masc.) -> τους
I hope that this helps. :-)
In ancient Gk the object of ακούω could be in either the accusative or genitive. When + accusative = to hear, ακούω + genitive = to listen to (often). Is there still a subtle distinction or has ακούω + genitive disappeared?