Hi there, you're not giving much info. What is your set up? How are you making the mixes - are they being recorded ready-made, or are you mixing it up on turntables, or DJ software etc.?
Free? Try Audacity - l sometimes use it for recording "What U Hear" audio being played on my PC.
Fiddle around with the input and output sound engine selector from the pulldown menus for each. Make sure to click monitor for each permutation, until you see that green bar bouncing from left to right, indicating that sound is coming thru into Audacity.