In Library, under My Music, you will see a section called "Sync Music". Click that arrow and wait until the music that is available on your phone populates (it may take a little while to load) After it loads, to sync that music to your VoiceShare account, simply select the individual tracks that you would like to add and click Sync.
If you already have music synced, click "see all" to see all tracks that are currently synced to VoiceShare (it may take a little while to load). Then if you wish to add more tracks or remove some, click the circular arrow icon in the upper right corner to pull up all your available tracks.
If you're Synced music is visible but unavailable in your VoiceShare app, it might be because you synced it while on a different device. Synced music will only play in-app when using the same device on which it was synced. If you switch devices, you'll need to sync your music again from music available n that particular device.
NOTE: iOS users, you need to have the iTunes Music app downloaded to your device in order to sync music.