It is a very simple fix.
Open GarageBand> then got to Preferences, you will see a box that is automatically checked "on" which says "export at maximum loudness". Check this box "off" and it should be able to export the song at a much higher volume.
Anyways, I have been using Audacity in my Windows laptop, as I haven't been able to install garageband on my laptop. Any ideas for the same?