even acapellas have bpm's you just have to able able to find them, try listening to the acapella serveral times over and count out the beats and phrases out loud a few times, it might take a while to find at first but its there. As you practice these more and more often you will be able to find the bpm alot easier and faster. Once you have found the bpm and where the beat would be if it had one its just like mixing in any other track.
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