The first time I tried substituting a GSB music file for one outside the game, GSB crashed on restart with an error message. Evidently the music does need to be in Ogg Vorbis format. Any music in MP3 format did not work for me, especially if all I did was change the file extension in the filename. It also crashed if I used an “actual” MP3 file. No quick fix there.
The “full” fix wasn’t a big deal, though. I simply downloaded a free copy of Audacity: a very nice open-source audio editor. I made sure to move the Quality slider to “maximum” in the app’s preferences. Then I imported an MP3 of my choosing, converted it to .OGG, and saved accordingly. The resulting file was accepted by GSB and played without problems. At least, no problems from the game’s point of view…
I wish that I had an uncompressed, lossless file to use as my source, though. A .wav or an .aiff file would have been ideal for me. You might want to rip an uncompressed file directly from a CD source.
I heard a noticeable loss in audio quality with the first file I ended up with - fairly muddy and indistinct sound. That was because I had left Audacity’s preferences alone… I think the default save value for Ogg Vorbis format is level 8. I had to move it up to level 10 (the max value) and re-save. I’m not fully happy with having so much music of interest for GSB purposes but only having copies in MP3. Going from one compressed format and then compressing it a second time is not ideal, even if the source MP3 has a high bit-rate. I used a 256kbps MP3 as my source, and even with my less-than-ideal ears I could detect a slight loss of quality when playing that music in GSB. I heard a small reduction in treble/brightness, and the bass was better than before but still a little “mushy”. True, It wasn’t a big disaster, but I really was surprised that I could hear the difference. I really didn’t think that I would – my hearing is definitely not the best. This may have been because I was using a Classical composition for full orchestra, which is going to have a lot to lose when compared to standard rock band stuff.
As they say, “your mileage may vary.” Just be sure to use the best quality source that you can find before converting to .OGG!