i have no idea why honestly the MP3 file is pretty loud but when imported into osu it just doesn't make a lot of sound anymore, the volume in the song settup is already at 100, any idéa what to do ?
The song's volume can't be set from within the editor, what you mean with 100% is the hitsounds setting.
Anyway, I suggest checking your volume settings in the game options, maybe you put the volume down there? If not, then it's possible that your mp3 is actually quiet, many music players (especially Windows Media Player) normalize the volume for songs to appear louder.