not sure why you want to know about the motherboard, but its based on Intel 925X, sound card needs ADI 1980 AC'97 Codec
plenty of info on dells website, if you search properly. for dimension 8400 overview
see here
as for the possible fix for the issue
quote:
If you experience poor audio quality, distortion, or echoing in Windows Live Call, try the following:
Run Audio and Video setup again
1. On the Tools menu, click Audio and Video setup.
2. Follow the instructions in the wizard.
3. When you reach the end of the wizard, click Finish.
Adjust your microphone
1. On the Tools menu, click Audio and Video setup.
2. Follow the instructions in the wizard. Use the slider to adjust the volume of your microphone.
3. When you reach the end of the wizard, click Finish.
If your broadband connection runs at 256 kilobits per second (Kbps) or less
1. On the Tools menu, click Options.
2. Select the Optimize my experience for a low-bandwidth broadband connection check box, and then click OK.
Notes
* If you are using USB 1.1, you may experience poor quality when you use both audio and video at the same time. You will receive the best quality audio and video when you connect your USB device directly to your computer.
* We recommend that you make computer-to-phone calls only over a broadband connection.
* If you are downloading large files or running other programs during your call, the quality of the call may deteriorate. Close other programs and file downloads to conserve bandwidth.
* Windows Live Call network performance can also affect the quality of your call. To check the status of the network, go to the Windows Live Call Network Status website.
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