Settings -> Scanner -> Edit Scan Paths
1. Press the top tight icon, a folder with a "+" inside.
2. By default the internal "emulated" root folder opens to show what Android allows you to see.
3. Press the left arrow <-- at the top of the list beside where the list shows "storage"
4. This should take you back one level and show the path to the SD card, usually a pair of 4 digit hex codes with a dash in the middle (i.e. 485E-1EE9)
5. Press that path name and continue to drill down into that folder to find your main Music folder, then hit "Select" at the bottom of the screen to add it to the scanner (repeat if you have more than one main music folder)
In general, a single "Music" or similarly named folder should have all of your music files. You can have additional subfolders that add layers for Artists, Year, Albums, etc. followed by the track info.
Good Example: {SD card address}/Music/Beatles/Abbey Road/01 Come Together.m4a is acceptable
Bad Example: {SD card address}/01 Come Together.m4a is NOT acceptable