How to enable stereo mixer in ubuntu?

Question

Audio

Good afternoon.
I have a video and I want to convert it to text.
On this site https://speechpad.ru/ there is such an opportunity.
You just need to turn on the stereo mixer.
The author shows how to do it in Windows.
And in Linux you have to do it yourself.
I found a solution through alsamixer, but this is just a tip.
How and where to turn the runners?
Thanks in advance.