Yamaha's solution on their website is to unplug it and plug it back in or not to use the USB connection and just use the speaker jack instead. Nice tech support. The solutions for Win2k/98/Me are almost all the same, but here is one of them (I assumed the 35D model):
I could not link the instructions directly... sorry.
http://www.yamaha.com/ycaservice/group002/fgrop002.htm
No Sound from YSTMS35D USB Speakers with a Windows 98SE / Millenium PC
Make sure all cables are connected properly. If the speakers are still not producing sound, it may be an indication that the device driver for the speakers was not installed properly. The installed device driver will need to be removed and the system rebooted to allow the computer to find and reinstall the correct drivers.
NOTE: If the following procedure(s) do not resolve the problem, an alternative is to disconnect the USB cable and the cable to the right speaker, and connect the Mini Stereo jack to the 'Speaker Out' jack of the PC's analog sound card.
Removing/Reinstalling the Drivers:
1) Click the <Start> button on the Windows taskbar.
2) Select 'Settings' from the 'Start' options.
3) Select 'Control Panel' from the 'Settings' options.
4) Double-click the 'System' icon.
5) Click the 'Device Manager' tab.
6) Click on 'Human Interface Devices' to view the list of devices.
7) Select 'Yamaha HID Audio Controls'.
8) Click on the <Remove> button.
9) Click on <OK> to confirm removal. 'Human Interface Devices' and other listings will disappear.
10) Click on 'Sound, Video and Game Controllers' to view the list of devices.
11) Select 'Yamaha YST-M35D USB AUDIO'.
12) Click on the <Remove> button.
13) Click on the <OK> button to confirm removal. 'Yamaha YST-M35D USB AUDIO' will disappear.
14) Click on 'Universal Serial Bus Controller' to view the list of devices.
15) Highlight 'Yamaha YST-M35D USB COMPOSITE DEVICE'.
16) Click on the <Remove> button.
17) Click on the <OK> button to confirm removal. 'Yamaha YST-M35D USB COMPOSITE DEVICE' will disappear.
18) Reboot the computer.
19) Insert the Windows 98SE, or the Windows ME Installation CD, as prompted, while the system is booting up. Windows will proceed with the device driver installation.
NOTE: The Windows Startup Sound should be heard as the booting up process is completed, indicating that the driver has been properly installed and the speakers are working. If the Windows Startup sound is not heard, and there are no other indications that the speakers are producing sound, then an additional procedure might be necessary to force the computer to reinstall the USB drivers from the Windows 98SE, or Windows ME, installation CD-ROM.
Deleting .sys Files:
1) Repeat steps 1 - 17 in the above procedure 'Removing/Reinstalling the Drivers' to remove the USB driver from the Windows System configuration.
2) Delete the following files:
c:\windows\system32\drivers\hidusb.sys
c:\windows\system32\drivers\hidparse.sys
c:\windows\system32\drivers\hidclass.sys
c:\windows\system32\drivers\usbaudio.sys
NOTE: These files may have become corrupted and will be recreated when the driver is reinstalled.
3) Repeat steps 19 - 20 in the above procedure, 'Removing/Reinstalling the Drivers'.