Hi
If, for example you have a videoclip on track 1 with the sound on Audio track 1 and you have an added soundtrack on Audio track 2, and then you have split all three tracks in error having meant to keep Audio track 2 complete, all you need to do is.....
Delete the left hand bit of Audiotrack 2 (leaving a gap). Now select and then grab the left end of the remaining Audio track 2. A thick yellow bar will appear. Drag this to the left across the gap. It will fill in the gap with the original sound again and there will be no split.
This will work with both sound clips and videoclips.
It's best to imagine that even though you seem to have split and even removed a portion of a clip it remains complete "underneath" so to speak and you can drag it back into sight. If you have in fact, during your editing "closed the gap" on any removed bits, then dragging them back into view will still work OK but will create a gap in any lower soundtracks and these would need then to be closed.
Nat