If Lenovo is not going to release drivers for you, you can find out what sound card is used on your computer and try drivers from other vendors that use the same sound card.

You can find more solutions in the following thread:
Missing sound recording option "Stereo Mix" / "Record What you hear" / "Waveout mix"

Or you can use software that doesn't need Stereo Mix for recording:
Audio/video recording software that allows to record sound without WaveOut/Stereo Mix