find_next_free_scb
src_parent_scb = find_next_free_scb(chip,mixer_scb->sub_list_ptr);
parent = find_next_free_scb (chip,ins->record_mixer_scb->sub_list_ptr);
parent_scb = find_next_free_scb (chip,ins->master_mix_scb->sub_list_ptr);