Alesis multimix 4 Noise

I bought lately the Alesis multimix 4, it’s brand new, but when I try to record something through the mic by line input I have noise from the track I’m playing on my computer. The enviroment is all silent and I play the track through my headphones. It doesn’t come from the headphones nor the speakers, just through the mixer to my vocal track, please help. I’ve tried to record on reaper and ableton, but still it doesn’t work.