How to switch off sounds of modems ?

Hi. Please help me with the problem..
I have few external modems connected to the dial-up server with the help of MOXA-LPT hub, i.e. they are connected not separately but together with one big port. The later provided by inner MOXA board connected to the PCI on the dial-up server. So The dial-up server installed under the REd Hat 9,0 OS. And the problem is that I cannot switch off sounds of these modems in the case when they connected with the port I have mentioned. So help me to switched they off. I know how to do it in the case when they are connected to the server separately, but in this case I don’t know how do it…