Judy,
Mike Young, our resident lawyer, may have a say in this. I'm not a lawyer, but I know that, if you do have audio testimonials, it depends on how you gathered them. Often, you can get those calls associated with a letter or email from the person stating "this is me." (Some testimonial gathering processes online now have that, like mine for example:
Testimonial Entry Form - Step 1 using audiogenerator and a phone).
But make sure to ask the people to identify themselves clearly when they leave an audio testimonial. If they don't, just ask them to record, in their own voice again, something that identifies them.
Also, make sure you have the standard earnings disclaimers on or near the copy stating that those testimonials are atypical of most results, and there are no guarantees of achieving any results at all.
Finally, another tip is to get those testimonials transcribed and "approved" or "endorsed" by the testimonial givers. You can send them a copy of the transcribed audio testimonial they made, and have them sign it.
Personally, this is going a little to the extreme. But as long as you can identify who is giving the audio testimonial, and, if the alphabet agency does ever a check, you want to make sure the person says, "yes, that's me." Problem is, with audio, you run the risk of them saying "no." For whatever reason (say, if they ever become disgruntled customers for whatever reason).
So if they identify themselves clearly on the audio, or back it up with a written approval, you're safe.