Lenovo laptop low audio volume

I love lenovo/ibm laptops. Specifically their business line. I've used T series laptops for quite a while. I don't know if this is a lenovo-specific issue or not, but web searches over the years seems to indicate at least that a lot of lenovo laptop users share the problem: low audio volume, and an inability to turn it up to reasonable levels.

I've suffered this in Linux for years and years. There seem to be a lot of fixes proposed by people: kernel parameters, manual ASLA configuration tweaks. They never work for me. So I tend to use headphones for myself, or external (amplified) speakers if for others.

Lately I've been booting Windows on my laptop (T521)... mainly for Skyrim. And suddenly today I noticed the audio volume, which had seemed fine up until then was annoyingly quiet in all software. I have no idea what could have caused this. All the mixer controls were maxed, and still it was a struggle to follow speech if there was any amount of background noise in my environment.

Searching around I find that Windows people seems as baffled as Linux people by this behaviour. Lots of wacky suggestions. A lot of people saying the suggestions didn't work for them.

Finally I found a really simple solution (this is Windows 7, but other versions have similar controls):

Control Panel -> Hardware and Sound -> Manage Audio Devices -> [Playback tab] Speakers -> Properties -> Enhancements [tab] -> check Loudness Equalization -> OK. 

And suddenly I have all the loudness I could want. Why? How!

The description of this option is: "Loudness Equalization uses understanding of human hearing to reduce percieved volume differences." It reports itself as being provided by Microsoft. What does it really do? How does it magically make the volume in all apps suddenly decent? I can't believe it's just software amplification, but perhaps it is. It's hard to believe the description given has the global effect of "fixing" the volume issues.

Furthermore, what happened to make the previously "okay" volume suddenly so weak?

I really have no idea. Is there some equivalent for Linux? I don't know. Whatever it is, it works on this Lenovo box under Windows... like magic. I don't like magic, when it comes to computers. I want to know what's going on.