Download
audacity and import your audio file into it. Next go to edit > select > all (or just press CTRL+A) and then go to effect and you should see something that says "Amplify...". Click that and use the slider to set the new peak amplitude to a negative value (you can use the preview button so you don't have to keep opening this) until it sounds like it's a reasonable volume then click OK. After that go to file > export audio and save the audio file as an .mp3/.ogg (these are the formats that are used for actual music, but note that to export to .mp3, you'll need the LAME encoder which can be found
here) or you can save it as a .wav (this is the format typically used for sound effects/short sounds, NEVER use this format for full length songs, they will have a very large file size and can cause gamemaker to crash).