I would suggest you invest in a pro audio card. A good start would be something like in the delta audio, like this one
http://www.musiciansfriend.com/srs7/g=h ... id/701345/
I think this card comes with 4 ins and 4 outs so what ever software you are useing you can assign a set of outputs to go out to your mpc.
now if you dont have the money then you just need to get a 1/8 spliter .
this has an 1/8 headphone plug on one end and rca lft and right on the other end and you can get 2 rca to 1/4 mono cables and input it into your mpc so this way you can sample the instruments from your computer to the mpc. but this way will degrade the sound comming out of you pc because the standard pc sound card have the most shitty converters in the world,,, unless thats the type of sound you are looking for.
im sorry , the above product I think does not have midi im not sure but i ment this card
http://www.midiman.net/products/en_us/A ... -main.html