"Why don't we hear?"
Translation:למה לא שומעים?
I can understand the confusion in this sentence. However, for example, if you are sitting with your friends in the living room and you turn on the TV but still, there is no sound, then one of you might say "למה לא שומעים" - when he actually means - "למה אנחנו לא שומעים" - since this sentence is in present-time, and since it's obvious that we are talking about US, then we don't have to use the "אנחנו". But again, it wouldn't be a mistake to use it.
I was taught at school that מדוע should be about the reason, whereas למה about the purpose. As ItaiHadas wrote, מדוע is quite high language; I'm not sure how many people who use it know the distinction we were taught at school. Also the distinction between reason and purpose is philosophical and can be turned around.
In Australia (and I'm guessing in Britain as well), 'Why don't we hear?' is almost a nonsense sentence and we would never say that. We would say 'Why can't we hear?' Or else you would need a word after 'hear', such as 'Why don't we hear this/that [whatever]?' Even then, it would be much more natural to use 'can't.