You could try to place a microphone next to your speakers, but that's not an ideal way to record that audio. Stereo Mix will let you open a sound recording program on your computer, choose Stereo Mix as the input and it'll record whatever sound comes out of your computer.

What happens if I enable stereo mix?

With “Stereo Mix” enabled, you can open up your favorite recording program and select that instead of your microphone before you record. ... In order to do that, you'd open up your audio recording application of choice (like Audacity), and find the option for microphone input.

What is stereo mix for?

What is Stereo Mix? Stereo Mix allows you to record exactly what was being output to your speakers, without going through any analog/digital conversion. This feature is very handy for recording the audio off a website.