If you use a current limiting resistor to both sides of the balanced mike line as, Bill AD5OL suggests, this will work. However, and very important, make certain that there is a DC blocking capacitor in series with the audio input so you do not send DC back into that circuit. I do not know what the input of the SDR looks like, but you do not want to pass unwanted DC into a circuit that does not want to see it!!
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