At the weather office
Weatherman: Yes, what is it?
Man: Are you the weatherman?
Weatherman: I guess so; sometimes I wonder.
Man: Oh, good. So, you can tell me what the weather’s going to be like.
Weatherman: Oh, I can tell you what the weather might be like.
Man: What it might be like?
Weatherman: …
I have two questions:
1- What does “sometimes I wonder” mean in this dialogue?
2- What does “Yes, what is it?” mean?
Thank you
Video link:
This is basically a greeting. It acknowledges the person and asks what they need. Similar greetings might be:
Yes, may I help you?
or simply
May I help you?
Yes, what is it? - This might be slightly rude or impatient, depending on the tone of voice.
“sometimes I wonder”
This means he is doubting if he is really the weatherman. He may not be literally doubting it though. It’s more of a comment about his frustrations with the job. Maybe he thinks his job should be more than it is, maybe with more prestige or respect. Maybe he feels that people don’t listen to his weather reports. Whatever the reason, he is expressing frustration.
Thank you so much, NearlyNapping 
A person who uses ‘What’s it?’ in place of "May/Can I help you?’ cannot but say ‘Sometimes I wonder’ especially when the weather forecast used to be (thanks to technology for today’s possibility) a prediction of ‘probability’ which would mostly fail!
Thank you so much, Anglophile