The thing with being an outsider to someone's day is that only ever see a small portion of it and its normally out of context to everything else. There could have been loads of reasons that the gran was ignoring it:
1) she was deaf and couldn't hear
2) the kid is deaf or hard of hearing (temporarily or permanently) and can't hear how loud they are
3) the kid has been a little devil all day and the gran is picking her battles, and you know a kid singing loudly is much better than them just screaming
4) the kid has had a near death experience that day and the gran doesn't care how loud he is being as long as he is making noise
5) she doesn't care
6) she is kidnapping the kid
7) she cares but can't control him because she isn't his mum.
8) she spent the last 20 minutes trying to get the kid to be quiet and it hasn't working so she is rushing to get the last of her shopping done.
9) the kid has its own mind and nothing, not even bribes will shut him up.
10) it's a kid being a kid
Loads of reasons, none of them probably true but since having a child I really try hard not to judge but I will try give a sympathetic look, because you know, there always has to be a naughtiest kid in the room/park/shop and one day it will be mine.