> That's why I don't get it, why does it sounds right on Spotify, but not on media players ? I get that the songs can come from different sources, and I may have some shitty albums rips for sure, but honestly I don't have just one song that have that problem.
I'm not sure if you do get the bit about different sources (or, at least, different MP3 compression factors).
MP3 is a lossy compression format; it reduces the size of the file by throwing information away. If you have a highly-compressed MP3 (e.g. 128kbps), so much information has been discarded that it sounds muddy, especially in the upper registers. If you compare that to something streamed from Spotify, you need to look at the bitrate that Spotify is sending; if it is streaming 320kbps, you should expect to have a much better experience, as more information has been retained. You would also need to look at the algorithm being used for the MP3 compression, and the options selected; there are many MP3 compressors out there, not just one that everyone uses.
You may even be using a lossless stream from Spotify.