with this package you can save an audioclip to mp3 in unity3d It works with both Windows and Android
Usage:
EncodeMP3.convert (AudioClip clip, string path, int BitRate);If any errors occured you need to change your .NET API level in unity build settings.
For example for Android: File => Build Settings => Player Settings => (in Inspector) Other Settings => Optimization => Api Compatibility Level : .Net 2.0