You can increase the volume of recordings by raising the microphone volume in the Windows settings.
If you are using Windows XP, open the Windows Control Panel. Double-click the Sounds and Audio Settings icon. The Sounds and Audio Device Properties window opens.
On the Volume tab, in the Device Volume area, click the Advanced button. The Volume Control window opens.
Look for a column with the title "Microphone". If the Microphone column is not visible, open the Options menu and click "Properties", and check the checkbox next to "Microphone", then click OK.
In the Microphone column, drag the volume slider higher. This will increase the volume of all recorded audio on your system.