Audio files are a great way to incorporate lectures, introductions, or various means of instruction. I have used audio recordings for introductions to some of my online courses. If I feel that there is something important and too difficult to put into a text file, I will record the information and then post the file in the home page of the course.
I have recently started to use a Voki character to add to a creative audio or "voice" to the coursework. This is a bit more unique than a recorded audio file alone. My Voki character is on the student Wiki site for their brachytherapy project.