To add background music from a library to your content, just drag and drop it on the Stage. It will not be visible there, but appear on the Timeline, right where the Playhead is (the vertical red bar):
1) Find sounds and music in the Storyblocks or Shutterstock libraries: Moovly offers you more than 100,000 royalty-free* sounds and music tracks in the Storyblocks library and even more (payable tracks) in the Shutterstock library. Open the Extended Library View by clicking the expand icon:
Make sure to select the Storyblocks or Shutterstock library and enter a search term for sounds (e.g. "background" or "applause" or "soft music". Then filter your search results for Sounds:
When you have found your favorite audio to use in your project, select it and click the "Insert in project" button (bottom right).
Once you have added a sound, it will appear in the Timeline. You can change its timing, length or volume and even apply Fade in and out effects.
2) To modify the volume level of your sound, first select the sound in the Timeline (its background turns blue when selected). You will then see a Volume slider under Object Properties on the right side of the editor. Use this slider to select the volume level you want:
See the "How to edit sounds" FAQ for more details.
*Royalty-free means that the audio content is provided under a royalty-free basis. In other words, you don't have to pay the composer a fee to use the content in your projects. However, certain industries (broadcasts are one) are governed by copyright/performance rights organizations (PROs). If the music composers also registered their music with a PRO, then a royalty must be paid to the PRO by the company under a separate agreement. In short, our royalty-free license does not cancel out any PRO obligations.
It is therefore required to comply with the regulations of your local/national PRO. More information is available on the AudioBlocks website (AudioBlocks is that part of the Storyblocks offering that includes sounds, loops and music): http://support.audioblocks.com/customer/en/portal/topics/610636-licensing-faq-s/articles?b_id=3253
See also https://www.shutterstock.com/license for the Shutterstock terms and conditions regarding the rights and limitations in the use of audio files.