I think "people we know are lying to us" is quite specific, so it should be "listen to THE people" with definite article "the", isn't it?
Actually, the definite article "the" makes the object more specific. So saying "the people we know are lying to us" is more specific in referring to particular people who you know are lying to you. Without the definite article here, this could refer to these types of people in general. However, this sentence sounds grammatically correct both with and without the definite article "the" and could be used either way.