O seguinte tópico tem muitas explicações e soluções alternativas:
Um deles é:
Since Microsoft intentionally removes the Stereo Mix / Waveout Mix option from its drivers, you need to try to download the latest version of the sound card driver and install it. If you have an onboard sound card, go to the support section of the manufacturer of your PC (such as Dell/HP/Toshiba/Sony/Fujitsu/Lenovo/Acer) and download an audio driver for your model of the PC that works with your Windows version. Please note that the manufacturer doesn't support Windows 7, you can try to use Windows Vista driver, althouth there is no guarantee that it will work and will not damage your hardware.
Outra observação é:
The problem is that you won't be able to enable the "Wave Out mix" ("Stereo Mix", "Record What you hear") on many computers with built-in sound cards. But you can use software that doesn't need the "Wave Out mix" ("Stereo Mix", "Record What you hear") recording option to record audio from your sound card:
Direct sound recording software allowing to record audio without Stereo Mix/Wave-Out Mix/What U Hear
- Audacity (Open-source) - sound editing and recording program (Windows 7, Vista or XP) (see instructions)
- Freecorder (Freeware) - sound recording program (Windows 7, Vista or XP),
- Replay Music - shareware sound recording program that can split and tag songs automatically (Windows 7, Vista, XP or 2000),
- Replay AV - shareware stream recorder that is good for scheduling online audio recordings (Windows 7, Vista, XP, XP, Server 2003; x32 or x64),
- Replay Media Catcher - shareware stream recorder that can record audio from your sound card as well (Windows 7, Vista, XP, 2000, Server 2003),
- Ask & Record Toolbar (Freeware) - sound recording program (Windows 7, Vista or XP)
- All Sound Recorder - shareware sound recording program with a scheduler (Windows Vista or 7)
Screencasting software allowing to record video with sound even if you don't have Stereo Mix/Wave-Out Mix/What U Hear
- WM Capture - shareware screen recorder that allows to record both audio and video (Windows 7, Vista or XP),
- Replay Video Capture - shareware screen recorder that allows to record both audio and video (Windows 7, Vista or XP),
Virtual Sound Card software
- Virtual Audio Cable - shareware software that creates a virtual audio device (Windows 7, Vista, XP, 2003; x32 or x64)
- Virtual Audio Streaming - shareware software that creates a virtual sound card (Windows 7, Vista; x32 or x64)
As you can see, the programs work not only in Windows XP, but also in Windows Vista and Windows 7. In fact these programs can be the only way to record audio from your sound card.
Freecorder4, Replay Music, Replay AV, Replay Video Capture, Replay Media Catcher, Ask&Recorder Toolbar, WM Capture use a universal audio driver allowing you to record sound even on those computers that do NOT have a sound card!