This application lets you download voices from Acapella group (same voices that Natural Reader uses) using a credit system. Each credit is $14.99 USD, at the moment all voices require one credit for purchase.
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Pro 30 day trial period per download
It is a luxury to be able to use a voice for 30 days in real-world use.
If you're confused when you get "This is an evaluation version of Acapella's Group' text to speech synthesizer" when using the newly downloaded/installed voice, look for the action button on the far right column for the voice downloaded/installed and in the drop-down menu click "start demo period".
Pro Has many voices in many languages
Adult, children, and some variations of the same speaker, for example Will a US American voice has from afar, up close, happy and sad variations in addition to the original voice.
Con Not multi-computer friendly
It is a PITA to deactivate a voice on one computer just so you can use a voice that you've purchased with your account on the same account but on a different computer in an attempt to not pay twice to use the same voice on two computers. Users have to make that choice, deal with the hassle of deactivating/activating voices or pay multiple times for the same voice for however many computers you use.
Con Have to fool around with permissions to install voice
For a supposed "manager app" it should deal with permissions by itself and not depend on users knowing how to fool around with that.
In Finder right click on the Library/Speech folder > "Get Info" > click unlock icon bottom right corner of get info window > enter administrative level credentials > give groups "Wheel" & "Everyone" Read & Write permissions > click the small gear icon and apply to all enclosed items > Install your voice(s), trial or otherwise, then set permissions for groups "wheel" and "Everyone" back to Read Only > click gear icon and apply to all enclosed items > click the lock button and reboot your computer. Voices should show up in system prefs accessibility > speech section. Might have to enable them by clicking customize.