For Windows XP, normal operation may not be guaranteed because Microsoft has ended support for this operating system. (Reference)

Double-click the Sounds and Audio Devices icon in Control Panel.

The Volume Control window will appear. Click Options, then Properties.

Select Recording, then under Show the following volume controls, check "Stereo Mix" and click OK.

If the sound is muffled, lower the Stereo Mix volume. If the sound is too quiet, increase the Stereo Mix volume.

Since Stereo Mix is a feature provided by the sound driver, on some sound drivers the sound may still be quiet even if the Stereo Mix volume is increased. In that case, you should also increase the system volume.