Use player imported songs?

a good way to go about that, is using StreamingAssets, so, basicly, you have to create a folder in the root folder of your project (/assets) called StreamingAssets. as you would notice, when you build the game, you will see a StreamingAssets folder in GameName_Data folder when you have the build. everything you put in the StreamingAssets folder will also be put there when you build the game. the issue is that you cant easily acces those files (you cant drag and drop the mp3 into a GameObject in the scene) You will need a special script. to locate the MP3 file, and set the Audio source file to that. Heres the Doc: (https://docs.unity3d.com/Manual/StreamingAssets.html)

i hope this helps you. Goodluck!