Nobody posts gently recorded music directly for customer use. You are probably experiencing compression/limiting/loudness processing common to for-pay music.
perfectly mastered recordings
Only because you haven’t heard the raw performance. I think it’s a good bet you would be shocked if you heard the original studio tracks on some songs before days of studio processing and boosting, late nights and many cups of coffee/tea.
There is a pre-packaged loudness booster. Chris’s Compressor was designed so he could listen to opera in the car. It evens out phrase by phrase volume variations and makes the whole thing louder.
I boost the first variable, compression, from the default 0.5 to 0.77.
If you don’t use a pre-baked utility, then you get to become an expert on the individual compression tools and variable adjustments. I don’t remember why “Limiter” occurs in two different tools. Sorry.
http://manual.audacityteam.org/o/man/compressor.html
http://manual.audacityteam.org/o/man/envelope_tool.html
Koz