Yildiz, I wish I could answer with a very strict one.
I received my a77 on Friday evening, so it's only 3.5 days. I'm still getting used to it and to the EVF.
On first impressions it's a nice beast.
First shots are excellent, I'm even happy with ISO 1600. Haven't tried with a higher ISO.
Is it worth an upgrade from a100. My answer would be a "YES".
For the type of shooting you mention, well, the answer would be a "most probably yes".
From my impressions (and not confined to: they say that..., or as John Doe said..., or http://www.canikonpreview.com
mentioned...), shooting in theatres, auditoriums, halls etc. with an a77 will leave you much more comfortable than any other Alpha camera I have used/tried, and that includes a100, a700, a550 and even a55 regarding the shutter sound. Some might disagree and mention NEX. But, mind you, I have said Alpha cameras I have used or tried.
Though a55's shutter sound is "low-volumed", a77 has a more "silent" one due to it's Front curtain shutter.
Noise wise, don't even think of it, because you have to compare it with your a100. I'll have to test it with a55 to tell you exactly which is better.
I would love to try on animals and see the results. And since I have not yet acquired any experience on that, I'll just keep my mouth shut (till I try of course
Night time shots? Have patience, and I'll let you know in a few more days. Same goes for video (btw, there are lots of friends here who are waiting for my video verdict, but I will rely on my son's and Anne's verdicts).
P.S.: if you love film then you will appreciate the a77 (my impressions... first impressions