The voices that Enhanced SMS & Caller ID use come bundled with your Android phone.
These default voices can be configured (change pitch, voice speed, etc) from your phones Settings.
If these default voices are too robotic for you, we have provided links to other voices, via the Menu -> More Voices (microphone icon), that you can download for a nominal fee.
These voices come from SVOX and after some configuration work wonderfully with Enhanced SMS & Caller ID.