Media Library Refresh
Introduction
If your newly added music does not show up, you can manually use media scanner to find it.
From toolbar, click "Rescan library" to start the music refresh. You can also make the scanning task running in background.
Music still not found
There are multiple reasons. Commonly your music format may not be supported. Please refer to following link to learn the supported media formats:
https://developer.android.com/guide/topics/media/media-formats.html
New music only
The scan process only discovers the newly added music files, which will not update existing music information.