First, I believe that you will need to hook up the sound to the TAD input on the Sound board. Next, I believe you go to the modem properties in the control panal and select audio output to soundcard. Finally, make sure that your TAD in is not muted. Also, many motherboards have modem sound inport jacks somewhere on the them (normally around the place where you would install the LEDs) I do beleive that by hooking it up to this jack, you can listen to the modem on the same channel as the wave sound, thus not using up one of your Analog inputs. Hope this helped you out.
If it works for you then don't fix it.