Why can't sound be reproduced from the speakers of the VAIO computer when sound is input through LINE IN or MIC?

Sound cannot be reproduced from the speakers of the VAIO computer when sound is input through LINE IN or MIC

SonicStage Mastering Studio cannot reproduce sounds that are input through LINE IN or MIC through the computer's speakers
However, sound can be output from the computer's speakers using Windows.
Please follow the steps below to reproduce sound input from the computer's speakers, through LINE IN or MIC:
1. Click Start and click Control Panel.
2. Click Sound, Speech and Audio Devices and click Sound and Audio Devices.
3. Click the Audio tab and then click Volume.
4. Check that the Line In or Microphone options are available.
If the Line In or Microphone options are unavailable, proceed with the steps below.
a. Click the Option tab and click Properties.
b. Click Playback. Select the Line In or Microphone options and click OK.
5. Clear the Mute options for Line In or Microphone. Close the volumn window and click OK.
* The volume output from the computer speakers can be controlled with SonicStage Mastering Studio.
Note: The volume adjusted with the volume control button is not effected during recording.