Put the first video or audio clip where you want it above or below the primary storyline, select it, and then Command+G to make it a secondary storyline.
Then (this is the key) select the secondary storyline, not the clip.
Then go back to the event where the clip is, select it, and hit the "E" shortcut. It will append the audio clip to the end of the secondary storyline where you want it instead of the primary storyline.
This also works for the insert function.
You can insert/overwrite any type of clip into video secondary storylines, but you can only insert/overwrite audio into audio secondary storylines.
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