When a file with both video and audio is added to a project, it is shown as a single video clip in the bin. By default, the audio is linked with the video clip. When the video clip is placed on a sequence, the linked audio is also placed on the sequence, and the two clips remain linked.
If two clips on a sequence are linked together, moving one clip will cause the other clip to move as well. This way, the video and audio will stay synchronized.
To unlink the audio from a video clip in a bin, click on the video clip and select Clip -> Unlink Audio and Video from the menu, or right-click on the clip and select Unlink Audio and Video. A new audio clip will appear in the Audio Files bin.
To unlink clips on a sequence, select the video clip then select Clip -> Unlink Audio and Video from the menu, or right-click on either clip and select Unlink from Audio or Unlink from Video.
You can also unlink or relink any (originally linked) clip by selecting it then clicking the Unlink or Relink button in the Clip Preview.
While linking is used to connect a video clip with its original audio, grouping can be used to connect two or more clips of any type. Grouping only works on clips on a sequence. Examples of uses for grouping include:
To group clips, select multiple clips in a sequence and select Sequence -> Group Selected Clips from the menu, or right-click on one of the selected clips and select Group Selected Clips. Select Ungroup Selected Clip(s) instead to ungroup clips.