To unsplit a clip that was accidentally split (either video or audio) you can simply stretch the tail end back to where it should be.
You have to use timeline view for this procedure.
In all cases select the appropriate audio or video clips by the usual method. The most common, simple case just involves selecting the clip, including the associated audio.
If it was accidental and is in two adjacent pieces:
*Select the portion of the clip to the right of the split.
delete it! You need to leave the original space.
*Then simply grab the right end of the remaining clip and drag it to fill the void. Done!
If it was intentional, or other cases where there is no space or not enough space following the clip:
*Use your favourite method of creating space.
](one of my favourite methods is to drag any handy clip that is longer than I need to where needed on the timeline and and then CLEAR it, leaving the space required. However the presence of multiple tracks can make life far more complicated than that.)
*Now that there is space (time) available, simply drag the right end of the previously split clip to wherever it should be.
*Finally use delete and close gap to get rid of any leftover space. Again, the presence of multiple tracks will complicate things a lot, so life is not always that simple.
In all cases be aware that any effects (with or without keyframing) will be based on the original segment of the clip. You may have to adjust keyframes accordingly.
The above applies equally to where the video and audio tracks are treated together and to situations where you have only selected the video OR audio (particularly applicable to accidentally having split a voice over or music over track).
Note: assumes clips are continuous. For instance if you used a "split" to bleep out a word, that is permanently two clips (use keyframing instead of split).