Does this mean that one can hear what is on the sea, or that one can hear when one is on the sea? If it is the latter, we could say, "everything can be heard at sea."
This. Personally, I've always seen at used.
Agreed, 'at' would make more sense
...men intet kan høres i rummet.
That guy definitely pronounces "havet" like "haven"... Is that the normal way of saying it? No "t" at the end but an "n"?