[All] How do you respond to someone who talks to themselves??

At work or in everyday life, there are many times when people next to you talk to themselves.

For example, what should I say to people who are making excuses like, "Oh my back...", "Ah, it's hot...", or "Oh my, what should I do?"

It's always awkward... Especially when it's someone higher up, it's hard to just ignore them, and I always end up just saying, "Heh heh..." I wonder if there's an appropriate way to respond.