Visible crew/equipment: When Dave first discovers that the chipmunks can sing, as they perform on the tree stump outside his window for him, after they sing 'Funky Town' there is a shot from outside looking through the window and to the right of Dave (through the window) you can see the long pole of the boom microphone; it is not seen in any other shots. (00:16:55)
Visible crew/equipment: After Dave has retrieved all of his musical equipment and instruments from the rain outside, there is a shot of him wiping his damp keyboard down with a towel. In this shot if you look in the glass door behind Dave, you can see the hands of a crewmember and the long boom microphone that he is holding, reflected in the glass hovering above Dave. (00:18:20)
Visible crew/equipment: When Dave wakes up from sleeping on his keyboard in the opening scene, as he straightens himself out and gets up to get himself ready for work, you can see the leg of a crewmember moving around in the glass door that is behind Dave. (00:03:05)
Visible crew/equipment: When Dave and the chipmunks are talking in the living room on Christmas morning, you can see a crew member reflected on the lamp shade.
Visible crew/equipment: When Dave enters the kitchen and finds the bowl of cheese balls, the chipmunks run and hide themselves under some bowls on the other counter. Then Dave leaves the kitchen and the chipmunks start to move along the counter whilst still under the bowls. The first time the bowls come to a stop (and Dave turns to look, etc) in the close-up as the bowls are coming to a stop, you can actually see the white string that was being used to make them move along; if you look then you can see it more easily to the side of the red bowl. (00:11:45)
Answer: No it isn't his voice.