You can no longer change the audio track of your video from within the YouTube mobile app. Use a computer to swap your audio track from YouTube Studio.
The YouTube Studio video editor lets you add music to your video from a library of licensed songs. These songs are from YouTube's Audio Library. You can use songs from the Audio Library in monetized videos.
Learn how to upload multi-language audio tracks to your videos.
Note:
- If your video has over 100,000 views, you may not be able to save changes to your videos. This restriction doesn't apply to users in the YouTube Partner Program.
- This feature is only available for videos shorter than 6 hours.
Add an audio track to your video
- Sign in to YouTube Studio.
- From the left menu, select Content
.
- Click the video you’d like to edit.
- From the left menu, click Editor
.
- Select Audio
use the search filters
to find a new audio track. Click Play
to preview tracks.
- When you find a song you like, click ADD. The song will appear in the editor in a blue box.
- Drag the box to change when the song should start.
- Drag the edges of the box to change the amount of the song that plays.
- Use the zoom options
for more precise adjustments.
- When you’re finished, click SAVE.
.
use the search filters 
to find a new audio track. Click Play 
to preview tracks.
for more precise adjustments.