Ah, i think i understand your problem. You have either your computer or your mpc hooked up to an amp and some speakers.
This is why mixers were invented. So you can hear more than one source at a time. You may have seen a mixer sometime, they're the big flat boxes with all the knobs and faders.
Maybe you still have an old dj-mixer somewhere, you could use that for now.
Good mixers are kinda expensive. I'm guessing you're just getting started, and maybe a good soundcard should be higher on your list that a mixer. You can mix in software with a lot of programs now, but you'll need a good soundcard for that. Once you'll get better, you'll need a way to record your beats too. This can be done in a computer, but you'll need a good soundcard for it. The standard souncard in a laptop is simply not good enough for this.
I'm sorry if i made things more complicated than they seemed, in short:
The sounds from Reason are not going to be coming from your mpc's outputs, ever. That's just not possible.
If you need more help, let me know.
