This happened to me once, from connecting the auxiliary device to an Aux Output cable rather than an Aux Input cable. The Beocenter actually amplified the cross-talk from the wrong pins of the DIN socket, if you turned the volume up high enough! This is why I always suggest buying a Tape In/Out RCA-to-DIN cable, because even if you get it wrong, the first thing you try is switching from the red/black RCA pair to the yellow/green RCA pair! (Yes, a genuine B&O cable has little plastic “cuffs” riding the 4 connectors, labeled “IN” and “OUT”. But the colors on the aftermarket cables are pretty obvious and about 10 times cheaper!-)
[Edit: If your laptop worked — if using the same cable — then this isn’t the problem. But I bet you have separate 3.5mm->DIN and RCA->DIN cables, rather than some adapter doing RCA->3.5mm->DIN.]