You can add sound to your animations when exporting them.
You will see this session when you choose to export your animation in the format of a video. Tap "+" to add a soundtrack.
Choose a soundtrack from your iTunes music.
You can trim the soundtrack and add-in fade-in/fade-out effect.
In the "trim" mode, simply drag the end end of the soundtrack to fit in your animation.
Drag the little dot on the soundtrack to add the fade-in/ fade-out effect.
Before you export the video, you can choose the aspect of ratio. If you are uploading to Youtube, choose 16:9 to fit-in the Youtube player.
We also encourage you to use other movie editing apps, like Write-on Video or iMovie to add sounds in your animation. They are more video-editor bases and allow you to make more adjustments on the track.