Věřily is for all female subject but this sentence has an indefinite subject so it should be "věřili"?
Yes "věřili" is correct here, but "věřily" is not correct.
"is not INcorrect"
Why “an indefinite subject?” The subject is „you,” and it can be a group of female people.
In a “type what you hear” exercise, are you supposed to be able to hear a difference between “věřily” and “věřili”?
No, all similar exercises shall be disabled. There is no difference in the sound, it is purely an orthographical issue.