Thank you guys for your kind help.
The situation in the context was that I had to prepare the address before I knew the examiners' age, name or marriage status, though I knew the count of examiners were about two. I must show respect and find a safe way to avoid unnecessary mis-comprehensions or making listeners uneasy. The title(s) was/were spoken in face when it came to the presentation, so it might be not the same as in writing.
From the replies, I understand that:
* "Dear Sir, Dear Madam" is more used in writing than in speech. But if it's used, it's not unacceptable.
(The "Ma'ma" in my post should have been Ma'am. Sorry for that.
* "Esteemed XXX" (Esteemed Examiners, in my case) is appropriate and is helpful to relieve the headache of considering the sex, age or marriage status. But it might be overformaml when applied to a single person in face.
* For a single person, I might be able to directly look at him/her and greet. In this case, I call a gentleman "Sir", and call a lady "Madam". So I would say, "Good Morning, Sir. I'm happy to be here to ...", or "Good Morning, Madam. I'm happy to be here to ...". Will this be ok in this single-person case?
* Eye contacts and expressions, etc, are part of the presentation and integrated with the words used in speech. Together they will decide whether the address make others feel respected and comfortable or not.
* If I do know their names and title(seldom in such an exam's case - it's intended that you don't know them.), I can call them in the form of "Professor John", "Dr. Jane", etc.
* "To whom it may concerns" can do well, but might be too formal or distant in talk to my understanding.
Do I understand this correctly? Any place I should take notice of?
I'm happy back here after a while of AWOL.
Happy holiday seasons to all. Thanks