Hi Hank,
The only thing I can think of is the method you are using to couple the audio signal you wish to measure. Hopefully you are using a capacitor? To chose the best value would be with the formula Xc= .159/F*C "F" is in hertz "C" is in farads and in most audio work .047 mfd is common. Xc give the "resistive" value for a capacitor to AC. Again I am not sure how you would ever obtain great accuracy without conversion to a square wave with a consistant time duration. Again hope this helps. Best, Ed