James2250 wrote:
you can increase the volume of the mp3 on the left hand side
No, don't do that as it's likely going to cause problems due to clipping since you would just be guessing the peak level. Use the amplify function instead which is listed under the effects menu. So, to start, select the entire wavelength (ctrl+a or just click and drag across the whole blue thing) and go to amplify. A box will come up with an amplification number listed. Don't enter in a different number as it will automatically be at the highest value before clipping. Click ok and the wavelength should be visibly bigger. If you encountered a problem regarding that, then you probably just didn't select it.
When that's all done, export as mp3 making sure that you set the bitrate correctly: For osu!, 192kbps is the maximum and 128kbps is the minimum, but if the original mp3 was something like 64kbps, raising it to 192kbps here will just make the file size larger and not increase the quality. You can check the bitrate of an mp3 in any worthwhile media player.