if you plug the sound card [ U-Mix 44 ] into the speakers and get sound
and then plug the sound card into the MPC 2000 audio inputs and the samplers outputs to your speakers …and get no sound
then you have a problem with the samplers inputs /outputs
check your MPC audio setting
you can also get a usb to MIDI interface and MIDI map the MPC to control the 16 pad sampler in Mixvibes DVS
…which is way more powerful than just using audio straight from the MPC
i use a roland mv 8000 with DVS…which is a lot like your set-up
this is a old video but it will give you an idear how it works
[video]
http://www.youtube.com/watch?v=eEilgMkv5RA[/video]