As tardes and às tardes?
How do the Brazilians notice the difference about the pronunciation?
There is no difference in pronunciation. You must discover it in the context. If the related verb demands "a" (like "assistir a") and the following noun is feminine, then it has the accent.
In portugal we have a different sound for "AS"... it sounds similar to the "ush" in the word "Usher"
"as tardes" means "the afternoons" and "às tardes" means "on the afternoons". There is no difference in pronunciation.