Oh, I think I kinda misunderstood your question. I don't use the audio file feature, but if I remember correctly, there is a volume control for it. Just slide the volume down?
When you have the audio or video file live there should be a slider to the left that says "Audio Volume" if you put your cursor over it. Also controlling the volume level on the device the sound is coming from should work as well.
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Hope this suggestion helps.