Why does LIne In 3/4 music bleed into vocal channel one while recording?

I'm recording vocal music on channel one of Alesis Multimix4USBFX, singing to accompaniment via computer thru Line In 3/4, on Audacity. On playback, I hear the 3/4 music in the background of the vocal on channel one. Why is it bleeding over? Using a Mac. Tnx... Tom in Texas
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