how do I raise volume on multiple tracks simultaneously?

Then use the Master Track automation curves:

• From the main menu bar: Track > Show Master Track ⌘B
• In the track head of the Master Track switch the pop up menu to "Master Volume" and adjust locally the volume, if necessary. This will apply to all tracks in parallel.

To increase the volume of the whole arrangement by the same amount, use the Masetr Volume Slider in the lower right of the main window:

Regards

Léonie

Sound like you really need to remix. But you could just add gain in the the master track effects. And a AU gain if you have one or just a eq. With eq keep it flat and crank up the overall gain.

Personally I like some dynamics.

You have to be clear on what you to to achieve.

Normalize

is the procedure to have your audio file reach 0dB or the maximum digital level (all 1s) at its highest peak level (remember, we are talking digital audio here). If an audio file is normalized, says nothing about how loud it is! It is a technical term that guarantees that there is no "headroom" in your audio file. You don't need any additional app for that, GarageBand can do that automatically (set in the Preferences).

Loudness,

on the other hand, is the perception of how loud a track "sounds" regardless of its actual level. Everybody audio file is limited by the same 0dB of maximum digital audio level when you compare two audio files (sixteen digits set to 1 is the maximum level in a 16bit system). However how to achieve the maximum loudness is a skill that turned into a horrible horse race which pretty much ***** the life out of the recorded music. If you want to get your mix louder, then you have to learn how to use (and hopefully not mis-use) the skill of mixing with all the available tools (compressor, limiter, frequency spectrum, etc). There is plenty of material out there (Websites, YouTube) to start with. Ultimately it is a lifelong practice and experience.