It depends what Sound Blaster model you have.
Older models allowed you to set the IDE port address as well as the IDE IRQ via jumpers.
Later models allowed you to set this via a software-based utility called "Diagnose" and, within Windows (9x or later) it would set it automatically.
Some cards also had a jumper to either enable or disable the IDE port. First ensure that it is enabled.
I believe your options were (but not necessarily always available):
IDE port addresses:
1E8h
170h
168h
IDE IRQ's:
10, 11, 12 & 15.
I would recommend using either IDE IRQ 10 or 11 (depending on what is available).
Make sure you then select your Sound Blaster IRQ to be either 5 or 7 (5 being the default).
You cannot share the IDE IRQ with your Sound Blaster IRQ (as this would definitely cause a conflict).