This can be changed manually to a user defined directory by opening up the Menu Tab in ARCADE and navigating to Account > Content Storage.
Change your Storage Location by navigating to Account > Content Storage to your External Drive. (No need to create a new folder on your Drive, Arcade will create one for you called 'Arcade Content'.)
After copying your Arcade Content folder to your External Drive, you may delete your original content folder.
Here is the folder where sounds are stored originally:
Mac: /Library/Application Support/Output/ARCADE/ARCADE Content
PC: C:\ProgramData\Output\Arcade\Arcade Content