Connect the phone to your computer with a USB Cable. If the default connection mode is not set to Mass storage, open the Notifications panel and tap the USB connection type > Media sync (MTP). Your computer will prompt you with different actions to take. Choose to open the folder to view files.
Use the computer to organize your audio files, then create a music folder at the top level of the microSD card.
Use the computer to copy music and other audio files into the folder that you created on the microSD card.
You can use subfolders on the microSD card to organize your music files or you can simply copy the audio files to the top level of the microSD card.
Open the Notifications panel and tap the USB connection type > Charge only. Then disconnect the USB Cable from the phone.