An issue with Past simple and past continuous

To me, it seems that you could use any of the sentences below and be happy about it:

1. I'm sorry, but I didn't understand what you said. You spoke too quietly.

2. I'm sorry, but I didn't understand what you were saying. You spoke too quietly.

3. I'm sorry, but I didn't understand what you said. You were speaking too quietly.

4. I'm sorry, but I didn't understand what you were saying. You were speaking too quietly.

Now, if I were a betting man, I would pick the third one. But it's really hard for me to explain why to use this one over the others. I would really appreciate it if you could enlighten me as to why it's correct (or incorrect).

I know that usually past continuous has an "anchor point" in the form of past simple or a defined moment in the past (e.g. 3 p.m.). But here there doesn't seem to be one. The verb "understand" forces us to use the simple tense and it doesn't seem to describe a specific moment, while "said" seems to describe the whole sentence uttered by someone.