My oldest is 6 and just got his 2nd set of tubes today. He didn't really speak at all up until he started birth to 3 classes when he was almost 3 but his pediatrician didn't really say anything at all about his speech delay. Then he started kindergarten early with speech therapy b4 we put the first set of tubes in at almost 4 because none of it really helped. His speech improved dramatically but he was still having a hard time with word pronunciation and hearing. When he started kindergarten at 5 he started speech therapy again and was improving better than we imagined but failed his hearing test. When we took him to a dr and they did the hearing test there he passed but I should of known better. This year we didn't really notice that he couldn't hear all that well cuz his grades were awesome but he failed the hearing test yet again and failed the one at his new pediatricians as well. We seen a hearing specialist and a ear nose throat dr and today he got part of his adenoids taken out and 2nd pair of tubes because she told us that his palate didn't fully form at birth and is still soft and didn't wnat to ruin his speech anymore. Fingers crossed that this works for him. I just thought I would give you a story from a mom whose been there. Just watch her and speak clearly and loudly she will start to mimic you because she can hear but you won't know how well till she does start to say some words.
I think 15 months is alittle early.. My almost 17month old only says ma ma. And not directed at me.. Maybe if by 2 it hasn't improved I would call.