Hey, was just wondering, if I wanted to allow the player to choose a song from his/her own music library whether iOS or Android (I understand that the process would vary between the two), to play in game. How would I go about it?
Any answers, will be much appreciated.
Thank you.
Cheers!! 
You could just purchase a plugin, maybe check out Prime 31.
Otherwise, you will need to create the objective C native code to do this, and add a set of interface methods to drive that code from you C# scripts.
See Unity - Manual: Building plug-ins for iOS
There is a similar unity support page for Android.
supernat is correct. I made a simple music visualizer for iPad with Unity and needed the functionality you’re talking about but couldn’t find any plugins that do it so I had to make it. I published the plugin iOS Music Library Access. It allows iOS users to access their music library and play any song, even audio books, and supports playlists, but doesn’t work with android.
Hope this helps