The irig guitar interface has a lot of cross talk because it is sharing the ipad's headphone jack. Turn down the volume of the drum track to where you can still hear it but not enough to cause sound leakage. Or turn down the ipads volume a bit. this is the fault of the irig, not cubasis.
Better yet, invest in a better guitar interface, one that plugs into the 30 pin connector(ipad 1,2 & 3) or lightning connector(newer ipads).
iPad 2, iPhone 5, Mac Mini
iRig Keys, iRig Mic, iRig guitar, You Rock Guitar (midi guitar) Mobile In Guitar, GarageBand, Alchemy, Sunrizer, Cubasis, Music Studio, Amplitube, Mobile POD, MultiTrack, bs-16i, SampleTank, AudioBus, AmpKit, JamUp XT, G2M
Epiphone Les Paul Special GT
Music For iPad