Sound (and occasionally video) disappears in VSE newbie here.

I was editing together some clips in the VSE. All was apparently well, but coming back it this evening, I find that only two strips are audible and only a few visible. I can't think what I might have done by accident to cause such a thing. If it helps, the audible strips are soundtrack strips which I added last. No apparent rhyme or reason to the invisible video strips. I am running Blender 2.77 on Linux.

Help would be much appreciated if you have any thoughts. Can you think what I might check to fix the problem?
When you can't see/hear strips content any more it's usually due to moved footage location. When Blender can't access the originally imported footage anymore it will just show black/play nothing even the strips remain in place and will work again once the footage is back in its original location or once you refreshed each affected single strips source.