I asked this question about a year ago ..
The answer is yes you can, using Audacity (free download) but it's a lot more fun to visit the cepstral and festival web sites and use their collection of free demo voices to make a unique one. You will want to convert the file to ulaw format.
-----Original Message-----
From: Randy Ballard <kb3hjc@gmail.com>
To: App_rpt-users@qrvc.com
Sent: Sun, Oct 10, 2010 3:36 pm
Subject: [App_rpt-users] Voice ID
Is there any way to use the built in letters /var/lib/asterisk/sounds/letters and group them together for a voice id
--
Randy Ballard
KB3HJC