He mumbled something about being late.

[WilliamTaft]

An example sentence using "mumble" in Longman Advanced American Dictionary: He mumbled something about being late.

Does the man say sorry or make excuse?

 

[5jj]

It is not clear what he said.

 

[emsr2d2]

He mumbled so the exact words weren't clear. The only thing the listener could work out was that it had something to do with the speaker being late.

 

[Skrej]

It is not clear what he said.

Literally.

 

[jutfrank]

Still, there's a high chance that in context there is an implication that he is apologising or making excuses, or at least explaining why he is late.