when you search google about this you find a number of old threads from people with the same issue, but every recommendation is telling u to open it in audacity and literally just turn up the gain. so either this:
or this:
neither of these methods have ever worked for me. when i turned up the gain to the point where it made any difference in osu, it would start clipping, and even if that wasnt the case it would still kind of suck because its all relative and u basically just need to guess how much you should turn it up
since then i've learned more about audio mastering and im happy to say i finally figured out the shit that actually works to fix this. it can be done in audacity but u need to use an effect called Loudness Normalization:
basically just ctrl+a the file and then click on this. i've been setting the lufs value to -9.0 since that seems to be the standard for CD audio, and it works great. i've tried this on a few songs that i wouldn't want to map otherwise because they're so quiet ingame and it does exactly what you would hope for
posting this here because it might be helpful but curious to know if anyone knows more about this or has their own solution