I am trying to find out if it's possible to build a complete IVR application by cobbling together parts from open source projects. Is anyone using a non-commercial VoiceXML implementation to build speech-enabled systems?
I've tried JVoiceXML in the past and had some luck with it.
http://jvoicexml.sourceforge.net/
It's java of course, but that wasn't a problem for my situation.
You might want to take a look at OpenVXI, I believe that a number of companies that sell very expensive IVR platforms (such as Avaya) have based their voice browser on it.
You can check out Asterix - http://www.asterisk.org/ for an open source solution.
Voiceglue (http://www.voiceglue.org/) is an implementation of voicexml using openvxi and asterisk. It may be a good option for you, it is GPL licensed.
If you want to build an IVR and you're not married to VoiceXML, you might try Twilio. They have a simple XML syntax, an awesome REST API, and small-project-friendly pay per minute pricing which lets you concentrate on building your app and not building/hosting telephony infrastructure. I built an IVR app using their system a few days and it was a pleasure.