Try using Wavosaur (
portable).
You can open .ogg files with it, although it would be better if you have the .wav file from the music and use that.
You can then do the following: menu process->normalise-> (and then either -6db or custom)
Save your result as wav and convert it using oggenc.exe or another convertor.
You might also see that there's a volume option. The volume option is to change the volume dynamically. This means, that if you run it twice saying -1db, it will change -2 db. With normalise, if you run it twice with the same setting, the second time, the result will be the same as when you ran it the first time. This makes it possible to find a good setting, and be able to repeat the process another time with that setting and have it right, right away.