I just wanted to say I found how to have audio from one video (a-roll) run behind audio from another video (b-roll) and without using any third-party software. Open your a-roll clip, with the cursor at the beginning click on “Add Audio” and choose “add music at the current point…” In the file browser screen on the bottom right choose “Audio and Video.” This will make your videos appear, choose your a-roll and click “Open.” This will add an audio track of your a-roll to your video. You can now split your video and add b-roll over the original audio. Both tracks, your a-roll and b-roll, are heard together. On the “Edit” menu double-click either the video or the audio track and you can edit volume independent of each other. There still appears to be some syncing needed if you drop your b-roll and go back to your a-roll but this should get most people started. Hope this helps

When you add a new sound always check to see if your other sounds have moved – If they haven’t great if they do – Here is a simple solution – Just add your new sound to the end of your project – Then copy and paste it where you would like it to go – It won’t move any of the other sounds – I discovered this by chance