You could also us an identifier. You can create an arbitrary identifier to either open the directory for the audio file, or open the file. As an example, to add a reference to an MP3 file, I created an identifier rule as:
Code:
Key: mp3
Name: MP3
Template: file:///E:\Music\{id}
Adding "
mp3:Eva Cassidy\2000 - Time After Time\05 Time After Time.mp3" to the identifiers of a book adds an "MP3" link in the details pane. Clicking that opens the MP3 in the associated music player.