volume control

I have a client who replaced her sound card with a YMF-724 PCI Yamaha Sound card. Now her master volume control slowly decreases to off. She has scanned for viruses using Norton Systemworks 2000 and ran WinDR. The computer is a Pentium 233MMX with 64MB RAM, 6GB Fujitsu HDD, Spacewalker Hot591 motherboard and a SIS6326 8MB PCI VGA.