Since capella 7 it is possible to allocate different sounds to voices notated in one staff: Mark the staff in the SystemTemplate and select "Sound".
By doubleclicking the field "For voices:", more voices are added and each voice can get its own sound. Volume (relatively to the rest of the score) and surround sound can be changed here as well.
Please read more in the manual or Help menu, keyword "Sound".
This feature is not available in capella start.