That old comment was about a completely different audio file. I can even agree that it still doesn't sound much like a question (adjusting intonation isn't easy currently), but when you translate, you translate a written sentence anyway, and the written sentence has a question mark, which doesn't leave any doubts.
The hint says "letter (message)" because we don't want you to think that it means "letter of the alphabet", but rather "a letter containing a message".
The word 'message' itself is way too unspecific. Granted, every letter contains a message, but not every message is written as a letter. Especially these days.
We are writing a message. - Piszemy wiadomość.