Hello Sunil,

Some possible reasons why you may be seeing this:

1. Looking at the typical performance characteristics (in the datasheets) of both devices, for the DAC and ADC components respectively, there is a varied response depending on what frequency you are trying to pass. I'd bet that if you varied the frequency of the sine wave, you would see varying attenuation levels, based on comparing these basic frequency response plots.
2. The analog conditioning circuitry on the Audio Ez-extender, both for the DAC output and the ADC input (op-amp and discretes) will definitely have an effect on frequency response. I can't say what the effect is without modeling the circuit, but if modeled, it would be possible to obtain some idea of how this circuitry affects the attenuation of the DAC output and scales the signal to the ADC input.

If you would like more specific information on the converters (AD1938 and ADAV801) I'd recommend you contact Linear Applications Support through their email support system:

https://form.analog.com/Form_Pages/support/standard_linear/contacttechSupport.aspx

Hope this helps.

Regards,

Tom