Yes, they did add a new Theme Song with the Cats & Dogs expansion.
This is the Original Theme Song:
https://www.youtube.com/watch?v=F7wC-sbvhB8
This is the new Theme Song that came with Cats & Dogs:
https://www.youtube.com/watch?v=WSRgfZccF-g
The two are supposed to randomly rotate when you start the game. I get the Original more often than not, which I prefer. It just hits me as more peppy and exciting. The C&D one is called a calmer version and many love it, although I (like OP) do not like it for the introduction. I find it to be rather boring and would prefer that kind of calm music to be in build/buy, CAS, or something like that when I'm concentrating on something else.
Afaik, there is no way to disable it as it's written in the game code. I myself don't really think it's worth disabling the C&D expansion for, but that's just my opinion.