My songs I put on my Micro SD Card does not appear on Samsung Galaxy S5!
Use a file explorer (e.g ES file explorer ) and view the contents of your SD card and select and copy your music to the file path  on your internal storage at:

storage/emulated/0/Android/data/com.google.android.music

Then close and restart Google Play Music and go into the menu and select Refresh. Then your music should show up. 

Note: The file path may be slightly different depending on the phone but you'll be looking for the subfolder com.google.android.music