One of these days we will effectively figure out why serious organizations can't seem to do even basic audio leveling on prerecorded videos that they intend for public consumption.
Seriously. It's 10 minutes of work, at the outside. Normalize the audio to an average of -3 dB so that people can actually understand what's being said without having to jack up their volumes beyond all reason, and then write it back down for the next thing that they have to listen to or get the ears blasted by the next notification sound that comes along.
I've been searching for a music player app that does volume normalization for years, without luck. I would like to be able to listen to normalized classical music, for example, because the volume varies a lot in this kind of audio from quiet passages to loud ones. When listening in car or on the phone while outside it becomes hard to hear the quiet passages because of all the surrounding noise, so, normally, I'd raise the volume, but then comes the loud part and my ears bleed. Normalized audio also plays nice with listening to music on the feeble phone speakers. I kind of find it nostalgic to listen on such tiny speakers, like the transistor radios from a few decades ago.
What I am talking about is raising the volume of the quiet parts, not making all the parts of the track louder. I would listen at normal volume but be able to hear at a sane volume throughout the track. I think this can be achieved offline with the compand effect on sox.
Another listening experience improvement could be to compare the relative volume of noise outside (using the mic) with the volume of the music being played and slightly adjust the volume to keep it above the background noise.
In conclusion, it is necessary to consider the fact that listening on the phone and car happens in noisy environments and quiet passages are almost drowned if the user doesn't compensate. Why force the user with mess with the volume every few minutes when it can be achieved automatically?
Maybe a brave developer will champion this idea and release a music player or even better, an app to normalize over the whole OS so we could benefit from it while using other apps like YouTube.
As the sibling comment pointed out, you are most likely looking for audio compression ("dynamic range compression") instead of normalization.
I know many people want such a thing, though I wouldn't touch it myself (I like my music to have some dynamic range). But just FYI, there already exist music players that have compression plugins like rockbox and vlc, you might want to check them out.
Seriously. It's 10 minutes of work, at the outside. Normalize the audio to an average of -3 dB so that people can actually understand what's being said without having to jack up their volumes beyond all reason, and then write it back down for the next thing that they have to listen to or get the ears blasted by the next notification sound that comes along.
There's just no excuse for it.