how to use cubase as just a sound module.

is it possible to use cubase as just a sound module, while retaining the sequencing from mpc.
this is the set up

keyboard to mpc via midi
mpc to midi interface contrlling instrument play in cubase
also its all mtc up as well
but when i select midi on mpc, which enable me to either play from keyboard or pads the instrument i select. (cubase)
only one instrument in cubase plays all the different tracks recorded on the mpc.
eg piano trk 1 cubase
guitar trk 2 cubase
flute trk 3 cubase
different sequences

i select midi 1a on mpc
2a
3a for each channel on different tracks.
hit play on mp and only the guitar plays everything.

any help
thanx