This is a good question. It is almost no way to explain the phenomenon in Chinese language with le. If you don't grow up with the language and the society, there is almost no way to crack the usage of this le.

For me, the sound/phoneme of le serves first of all to indicate whatever is done or, secondly, to soften the tone of speech.

For instance, in your sentence, "教書的時候，聲音要大些，平時跟朋友說話，就不用太大聲了。", you may as well say, "教書的時候，聲音要大些，平時跟朋友說話，就不用太大聲。". But the latter would sound flatt and hard. With that 了, the expression is softened to be some kind of suggestion or an observation to be agreed with. Without 了, it sounds like a must: You must speak softly when you are not teaching.

Anyway, this is just my own language feeling. Other people mayhave their own observations.