Video clips in the Movie Window normally have synchronous audio. In the absence of a transition, both video and audio cut from one clip to the next. When a transition is placed between two clips, the audio cross-fades (the audio equivalent of a dissolve).
The only exception to this rule is the Fade transition, which takes the audio completely out then back in again.
Normal transitions cause a cross-fade in the audio (left). In a Fade transition (right), the audio fades down then up along with the video.