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If you mean a USB DJ Mixer, then try some of the freeware DJ programs such as: VIRTUAL DJ; CROSSDJ FREE; CUTEDJ. there are others as well.
Call yourself DJ mixer
mixer is when you ix the music only but he dj amp i for all the gear like speakers mixers turntables and stuff.............
The most economically successful DJ mixer in the DC area is Pioneer DJM-700. It has a four channel mixer and it is available at a price of 89 dollars.