“Do you hear” is present. So do you currently hear any whistling.
“Do you ever hear”, means any time in the past. There is an implication that the asker has heard whistling on multiple occasions and is wondering if you have heard the same. That is not always the case though.
The addition of ‘ever’ in the question indicates that the questioner wants to know if there is a possibility/chance that the other person heard the sound.
Another example - Have you ever been to America? Again is it possible by chance that you have visited America?