How to play mp3, mp4 files on the local disk

The short answer is, you cannot play certain media files with Unity without a proper plugin. You need a license.

You can however play ogg- and wav-files, as a small bandaid to this void of an answer, importing them at runtime could look like this:

 #pragma strict
 
 import System.Collections.Generic;
 import System.IO;
 
 static var path : String = "./"; // Is equal to where you have your executable
 static var fileTypes : String[] = ["ogg", "wav"]; // Valid file types
 
 static var files : FileInfo[];
 static var audioSource : AudioSource;
 static var audioClips : List.<AudioClip> = new List.<AudioClip>();
 
 function Start () {
     // If in editor the path is in Assets folder
     if (Application.isEditor)
         path = "Assets/";
     
     // Set an AudioSource to this object
     audioSource = audio;
     if(audioSource == null)
         audioSource = gameObject.AddComponent(AudioSource);
     
     // Find files in directory        
     yield GetFilesInDirectory();
     
     // Play a clip found in directory
     if (audioClips.Count>0) {
         audioSource.clip = audioClips[0];
         audioSource.Play();
     }
 }
 
 function GetFilesInDirectory () {
     var info : DirectoryInfo = new DirectoryInfo(path);
     files = info.GetFiles();
     for (var file : FileInfo in files) {
         var extension : String = Path.GetExtension(file.FullName);
         if (ValidType(extension))
             yield LoadFile(file.FullName);
     }
 }
 
 function ValidType (extension : String) : boolean {
     for (var validExtension : String in fileTypes)
         if (extension.IndexOf(validExtension) > -1)
             return true;
     return false;
 }
 
 function LoadFile (path : String) {
     var www : WWW = new WWW("file://"+path);
     var clip : AudioClip = www.audioClip;
     while (!clip.isReadyToPlay)
         yield;
     clip = www.GetAudioClip(false);
     var parts : String[] = path.Split("\\"[0]);
     clip.name = parts[parts.Length-1];
     audioClips.Add(clip);
 }

Here is one way to use library for playing link text

http://lmgtfy.com?q=Unity+Play+Audio+from+File