Summary:
- The sound card in the T420 and various related models supports stereo mix recording (record "what you hear").
- The drivers available for the T420 and other recent models do not enable this feature.
- Older drivers for older laptops do enable the feature.
- So, you need to download and install the older driver.
- But, the older driver is for a different laptop model and isn't configured to install on your newer model of laptop (such as the T420), even though the driver will work fine with the sound card.
- So, you need to reconfigure the installer for the older driver to know about the hardware ID of your particular laptop's sound card. Then the installer will run and you'll have the older version which enables this feature.
That's what most of the instructions in this thread are about. Most are then pointing to an older Windows 7 driver download. In that case, you are finding and downloading the "IN2AUD25WW5.exe" driver, extracting the setup files by hand with an unzipper (since otherwise that file automatically installs itself), and modifying the NELCC2wa.inf configuration file with a line like:
%HdAudioFunctionDriver.Pebble5071.DeviceDesc% = HdAudModelNELA0V,HDAUDIO\FUNC_xx&VEN_xxxx&DEV_xxxx&SUBSYS_xxxxxxxx
The part to the left of the comma stays the same, and should match a similar line that's already in the file. The part to the right of the comma is the part where you need to insert your particular sound card's hardware ID, which you can find in Device Manager->your Conexant sound card->Properties->Details
Then you update your driver, point to your extracted folder (which should ONLY have your modified .inf file in it, not also the original .inf file), and install.
But if you're using Windows XP, you can't use the IN2AUD25WW5.exe driver, which is for Windows 7. Instead, you need "6wd108ww.exe", which is a 2011 driver, v3.97.0.0, for the X100e. But it will work fine on the T420 by reconfiguring the installer to know about the T420 sound card's hardware ID.
Download 6wd108ww.exe here: http://support.lenovo.com/en_US/downloads/detail.page?DocID=DS000772
Run it to extract the files. (Unlike the Windows 7 installer, this one extracts for you but doesn't then execute unless you tell it to.) Then find where you extracted to (perhaps C:\DRIVERS\WIN\AUDIO), pick XP32 for 32-bit XP or XP64 for 64-bit XP, and modify the .inf file: FL3Peb5a.inf
You will find a section 43 lines down called "[Conexant]", and you will add a line like this:
%HdAudioFunctionDriver.Pebble5071.DeviceDesc% = HdAudModelFL3,HDAUDIO\FUNC_01&VEN_14F1&DEV_506E&SUBSYS_17AA21CF
The part to the left of the comma stays the same. (Note that this is different from the line used for the Windows 7 driver.) The part to the right of the comma should be changed to the hardware ID of your sound card. The one I've pasted is for my T420, so you can probably leave the whole line the same if you also have a T420.
As above, make sure this modifed .inf file is the ONLY .inf file in this folder (i.e. don't keep a backup copy hanging around in this folder). Then in Device Manager, update your driver, point to this folder, and install. Without even rebooting, you should now find Stereo Mix as a recording option:
Volume Control->Options->Properties, change Mixer device to "Conexant HD Audio input", and you should now have "Stereo Mix" listed above "Microphone." Check the Stereo Mix checkbox, hit OK, and you can then check the "Select" checkbox under Stereo Mix to record what you are listening to.
I tested my built-in mic after doing this (check the "Select" checkbox under Microphone), and it still works fine.
I hope this helps other Windows XP users, as well as users of various models of laptop, who might struggle with some of the earlier instructions that don't always explain which parts of the line in the .inf file need to be modified, or make it clear what needs to be changed between the Windows 7 and Windows XP instructions.
Now, if only there were a way to get a stereo line in on this machine without buying an external soundcard...
