First, I absolutely adore the waiters singing, that is the highlight of my cruise!
As far as "elegant night", I usually wear church clothes, because I'm not going to pack a formal dress to wear for a few hours. But I wouldn't wear jeans either. What is really ironic about this question is, I would have to get my pictures out to see what "I" wore on "Elegant Night" and I have no idea at all what the people across the table wore. But I sure did enjoy their company! That's what matters most to me! Just dress comfortable. Those precious cooks didn't cook all of this great food for nothing.
Loosen up, it's your vacation! Enjoy yourselves and your tablemates.