Do you have an oscilliscope that will measure audio frequencies ?
( perhaps a sound card scope would even work. )
You can determine what the signal level is coming out of the DAC.
If you read page 13 of the pdf for the Class D amplifier you will see  
you have the option of single or differential inputs.

If you want to mix the output of the DAC with an aux input you will  
have to add an op amp in my opinion.
MA


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