听说 hear something from unconfirmed source or eavesdropping by accident, you don't really know what happened 听说了 have heard something which did happen, but you don't care 听说过 had heard something from unconfirmed source or eavesdropping by accident, you can't make sure that you know the whole thing 听说过了 had heard something which did happen, but you don't care

我说下我的理解，看词组是放在哪个位置，放中间基本上是没有区别的， 使用频率也是按1234来排，例： 1.我听说你在问有什么区别 2.我听说了你在问有什么区别 3.我听说过你在问有什么区别 4.我听说过了你在问有什么区别 但是如果放句末就会有点区别 1.我听说XXXXXXXXXXX（后面一定会跟上一段句子） 而其他的三个语则可以直接做结尾 有点绕，我还从来没考虑过这个问题 看其他人补充。