Switching and Cutting Video and Audio Items Separately In the Final Cut Pro 7 manual is explained how to switch and cut video and audio items separately. At the end, it says:


  However, independently switching video or audio angles breaks linking
  relationships between a multiclip’s clip items in the Timeline. After
  you perform the switch, only the audio item whose angle you switched
  remains linked to the video item.


Which is exactly what happen in my case.

How can I switch angle for a piece of audio once the relationship is broken in the timeline?
You should be able to drag the original clip from the Browser into the timeline and replace the clip you're working with.

If you want to make sure not to mess up your edit, double-click the 'broken' clip in the timeline to bring it into the viewer, and make note of your in and out points.  Then open the original clip from the browser into the viewer and set the same in and out points. Now replace the 'broken' clip in the timeline with the new one from the viewer.

If you're asking how to switch audio and video independently without breaking the relationship, I'm not sure.