Connect the phone to your computer with a USB Cable. If the default connection mode is not set to Media device (MTP), open the Notifications panel and tap USB connected > Media device (MTP). Your computer will prompt you with different actions to take. Choose to open the folder to view files.
Use the computer to copy music and other audio files into the folder that you created on the phone.
To disconnect your phone, open the Notifications panel and tap USB connected > Charge phone. Then disconnect the USB Cable from the phone.