Traipse back to the Track menu and choose Show Master Track. As its name hints, this track allows you to impose volume, pan, automation, smart controls, and EQ settings over every track in your song. So, if you wanted to fade out all of your tracks, enable the master track, enable its automation, choose Volume from the pop-up menu, and click in an adjustment point where you want the track to start fading. Click in a second point where the track will completely fade out. Drag the second point all the way to the bottom. If you find that the fade is too fast or slow, just drag the first point to the left or right to change where the fade begins.
The Master track has its own set of presets. If the Library pane isn't showing choose View > Show Library. There will be at least a Factory entry in that pane. Select it and you'll see a variety of presets. Choose one and expose GarageBand's smart controls if they're not already on screen. As you select presets you'll notice that different controls appear in them — some have a reverb effect, for example, while others include an exciter effect. Find one that approximates the overall feel you want for your song and start making adjustments. You can also compare the current adjustment with the default by clicking the pane's aptly-named Compare button.
I swore I wouldn't get too wrapped up in the little things, but there are two additional features that may have no interest to you that I'd like to point out for those who want a slightly deeper dip into GarageBand. The first is keyboard sensitivity.
I'm a keyboard player and so have a measure of control over how hard I press down on keys. So, through pure physicality I can cause a piano to play quietly or loudly. Those who haven't spent a fair amount of time banging on the faux ivories don't have such fine control. These people may want a software instrument to play at the same volume regardless of how hard or softly they press a key. GarageBand makes accommodations for all of us.
Select a software instrument track, expose its smart controls, and then click on the Info button in the pane. You'll see a Keyboard Sensitivity slider. If you adjust it to the right, GarageBand will be less sensitive to how hard you bang on your controller. I know this seems counterintuitive, but what you're really adjusting is the interpreted velocity level. I know, I know, say what!?
When you drag the slider towards More, GarageBand interpets this as "make my playing more powerful because I have weak baby fingers." Drag it toward Less and it hears "Out of the way, dude, I can totally handle the nuances of my keyboard with my powerful phalanges." Of course if you drag the slider to either extreme your playing will be either too strong or too weak all the time. So, adjust according to your taste and keyboard controller. (For example, if you have a keyboard with semi-weighted keys and are accustomed to a piano's weighted keys, you'll want to drag that slider a bit to the left of Neutral as your keyboard offers little resistance.)
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