Despite the presence of VU-meter is much easier to control the sound in your ears. When sound is being sent to the Line-In, I can see the Input Level changing in sound recorder, but why I can't hear the sounds when I import some tape-recorded songs from a cassette-recorder that plugged on the Line-in jack? On Windows XP you can fix this problem by simply adjusting the volume controls on your sound card. With Windows Vista and Windows 7, it's a little more complicated.
Right click on the "Speakers" icon in the system tray and click on "Playback Devices".
Right-click on "Speakers" and select "Properties". Select the "Levels" tab.
Unmute "Line-In" input and increase level up to maximum value. Here you can also adjust the balance.
Done! Now you can listen to the sound from the "Line-in". If you want to record music in silence, just repeat over and mute this input again.