Export sound_extra to a separate sound file.

[execut4ble]

Currently it appends it to the video file making video and audio track lengths differ, this makes the clip unaccessible past the video track in programs like Adobe Premiere CC, possibly some others too.

Either separate the extra sound in another file or fill the remaining video track with last frame/black to match the length.

[YaLTeR]

Oh, another Premiere stupidity, what a surprise.

As a workaround, you can separate out the audio track using ffmpeg -i input.mp4 -map 0:1 -c copy out.mp4.