How to export garage band projects with the best sound quality?

Four years later, but I've been doing a lot of testing and experimentation around this topic of late and have made quite a number of discoveries.

1. If Auto Normalize is ON, the global volume slider makes no difference — unless the global slider is set to negative infinity db, in which case silence will be generated on export. Setting the slider to anything in the range of -78.2 to +6.0db and exporting with Auto Normalize ON will result in virtually the same file. Virtually, only in the sense that something weird goes on with auto drummer tracks. In every other way, though, the exported file is bit-for-bit the same.
2. When Auto Normalize is OFF, the global volume slider does play a very important role. Set it too low, and the exported sound will be soft. Set it too high and your sound will clip.
3. It's important to note that the meter associated with the global volume slider is incorrect; off by about 2.5db. I actually think this is intentional (though undocumented) on Apple's part, so that people will mix their tracks such to not "go into the red", but the overall loudness will be corrected on export as long as Auto Normalize is on.
4. I typically export with Auto Normalize OFF, with the volume slider set initially at -2.5db. I then import the AIFF file into Audacity where I can easily see if portions of the track are clipping. When clipping is present, I go back to GarageBand and decrees the global volume setting, then recheck for clipping in Audacity. Unfortunately, GarageBand's volume slider does not support granular changes, typically jumping from -2.5 to -2.8 to -3.0, etc... (with is absolutely horrendous UI design!), so it's difficult to find the absolute correct level at which GarageBand exports with Auto Normalize OFF will or won't clip. I usually choose the volume level where to clipping appears, and then add AUPeakLimiter to the Master track as the very last effect in the chain to catch any extraneous volume spikes. For me, this process seems to yield the cleanest sound when exporting from GarageBand 10.x

For best quality try to share the song uncompressed (AIFF).

How is your "Auto Normalize" setting in the "Advanced" preferences? Try , if changing this setting helps:

Also, check the global volume slider in the upper right corner of the window:

My screenshots are from version 10.0.1. What is your GarageBand version?

Regards

Léonie