How to download additional text-to-speech voices for later installation? I would like to download additional text-to-speech voices for OS X to later install on multiple systems or systems without an internet connection.

Is this possible, or do I have to be online to download them through the System Preferences ?
The downloaded voices are stored in /System/Library/Speech/Voices.

You can copy these between Macs. Download the desired voice on one Mac through System Preferences and copy it to the other Macs that you want.