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You can add a soundtrack to your TouchCast in the editor. After you've recorded your TouchCast, the editor will appear with your clips at the bottom of the page.
1.In the TouchCast Editor, select the clip that you'd like to add a soundtrack an audio to.
2. Tap "+ Add Audio"
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3.Tap "Select a Soundtrack"
4. Select the source of your soundtrack. You will need to log into your chosen cloud service account if you're not already logged in.
5. Select the song file you wish to insert into the clip.
6. You'll see a loading bar while your song is being imported. 
7. After the song imports, you can choose soundtrack settings for loop and volume level in the TouchCast Editor under the "+ Add Audio" tab.
this method adds sounds to entire video composition.
How to Add a Sound Effect to a particular touchcast or to a group of Touchcasts
- Select the particular touchcast in which you wish to add the sound effect.
2.Open in Editor.
3.Tap on +vApps and then tap on 'Effects'.
4. At the bottom right side you will see 'Sound Effects'.
5.Tap on ' Sound Effects' to expand its options.
6.Select any particular type of Sound Effect and then start recording.
7.Record again to record with the selected sound effect.
8. Hit 'Done' to save.
This way you can add some custom sound effect.
