You did not mention your Music Maker version any way if, you want to add audio files you got on computer or even on external drive in that case click on the File Manager of Music Maker then browse where your audio file is located. Choose the desired audio file drag and drop in the work space of Music Maker lets say such as Track one. After you drag and drop the audio file can move it to place you wish. When you are done please save the project.
Note: Before you drag and drop the desired file if you click on it you can listen to that too.
Please also note if your audio file BPM is different than the other it may prompt to adjust. Do as you wish. In Music Maker it is best if all audio files are of same BPM.