No. The actors do not provide any of the singing talent in any of the Chipettes, or the Chipmunks. As a life long Chipmunks fan, I can tell you exactly how all of the singing voices are performed. Ross Bagdasarian Jr, has supplied the talking voice for both Alvin, and Simon since he took over the Chipmunk empire, along with his (at the time) future wife Janice Karman. Ross Bagdasarian Jr, has provided the talking voice for Simon, but at most times during the animated cartoon series, and most of there soundtrack CD albums, he has also provided the singing voice as well. For the new live action movies however, he has handed over the mic to long time friend Steve Vining. Steve Vining provides the singing voice for Simon, while Matthew Gubler provides the talking voice. Ross Bagdasarian Jr however, continues to provide the singing voice for Alvin in the new movies, while his wife Janice Karman provides the singing voices for all three Chipettes, and Theodore.
So yes! it's actually Janice Karman who is singing all the single ladies in the Squeakquel, and providing the singing voices for the Chipettes, and for Theodore. LOL! And Steve Vining performing Simon, while Ross Jr is providing the singing voice for Alvin.
Janice Karman has a very good singing voice, but one of my favorite songs is the one she sings here→http://www.youtube.com/watch?v=_Y5GpYQSpVQ
Janice Karman also provides the talking voice for all three Chipettes in all of the animated cartoons, as well as there first motion picture movie, The Chipmunk Adventure.
Here is at least a little bit more information on Steve Vining if you wanted to know more about him.
http://en.wikipedia.org/wiki/Steve_Vining
If you have any more questions about the Chipmunks, feel free to email me.