Have a look at the workshop manual, available on burnkit2600.com - if you look at the circuit diagram around IC15/IC16 you'll see that these buffer the sounds. If you want to do anything cleverer than the four groups available at these chips, you'll need to look at replicating the bit with the transistor too.
Buffer the outputs with another opamp, and you should be fine.
Are you saying you just want to take the sounds from individual outputs so you can put different effects on them?
If so you can just take a wire from each of the individual output points on the vertical row of resistors above the Roland chip and solder it to the tip connection of a minijack socket. Solder the ground of the socket to the board ground and you've got individual outputs.
i'm assuming you've searched this forum for this information?