1) Press (WindowsKey)-R (to run a program)
2) Type regedit, press enter, and then click Continue (depending on your security settings)
3) Go to Edit - Find (or Ctrl-F)
4) Type EnableInputMonitor and click Find (or Find Next)
5) Double-click the EnableInputMonitor entry. It will come up as 0000 00. Change the 00 to 01 (so it's 0000 01). Do a Find Next just to make sure it's the only EnableInputMonitor registry setting on your system
6) Exit the Registry Editor and Restart the PC
6a) Wait for Vista's criminally long reloading time
7) Right-click the little Sigmatel Icon and select Playback Devices
8) Click SpeakersHeadphones and then Properties.
9) Under the Levels tab, there is now a new option called Input Monitor, which is muted. Click the speaker to unmute to Input Monitor, and the Line-In port now functions as expected I HAVE A SIGMA AUDIO TEL AND WIN VISTA HOME BASIC.. AND THE "EnableInputMonitor entry" WASN'T THERE SO YOU MUST CREATE IT

.. THIS WAY...
1) Open regedit (Start->Run, type regedit)
2) Navigate to HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Class\{4D36E96C-E325-11CE-BFC1-08002BE10318}\0000\Settings\filter\SpeakerHp\
3) Create a new Binary Value
4) Give it the name EnableInputMonitor
5) Set it's value to 01
6) Restart your machine
7) Open your Playback Devices (right click on volume control icon in system tray, select Playback Devices)
8) Open the Speakers/Headphones
9) Go to the levels tab
10) Unmute the input monitor