W3.org:

W3.org has 287 URLs listed in 163 Dmoz categories.

W3C: Voice Browser Activity
Portal of the activity of the W3C Voice Browser Working Group including public mailing list, links to current status, work under development, links to drafts and specifications, FAQs.
http://www.w3.org/Voice/
Top/Computers/Data_Formats/Markup_Languages/XML/Applications/VoiceXML

Voice Extensible Markup Language (VXML) Version 2.0
Pointer to the latest 2.0 specification including new errata published by the W3C consortium.
http://www.w3.org/TR/voicexml20/
Top/Computers/Data_Formats/Markup_Languages/XML/Applications/VoiceXML

Voice Extensible Markup Language (VXML) Version 2.1
The pointer to the latest 2.1 draft specifications including errata. Version 2.1 is a minor refinement of 2.0 with features that are simple, already implemented by some vendors, and not already available within 2.0 version.
http://www.w3.org/TR/voicexml21/
Top/Computers/Data_Formats/Markup_Languages/XML/Applications/VoiceXML

Voice Browser Call Control: CCXML Version 1.0
The upcoming call control language for more elaborate control than VXML transfer. CCXML uses VXML to specify dialogs with connected call parties.
http://www.w3.org/TR/ccxml/
Top/Computers/Data_Formats/Markup_Languages/XML/Applications/VoiceXML

Semantic Interpretation for Speech Recognition (SISR)
The specifications which describes how tags within SRGS grammars may be inserted to support basic post-processing or full semantic interpretation. This is then used by VXML to support advanced features such as mixed initiative.
http://www.w3.org/TR/semantic-interpretation/
Top/Computers/Data_Formats/Markup_Languages/XML/Applications/VoiceXML

W3C's Introduction to VoiceXML 2.0
A nice VXML tutorial provided by Dave Raggett to the W3C. A great introduction to VXML with code samples.
http://www.w3.org/Voice/Guide/
Top/Computers/Data_Formats/Markup_Languages/XML/Applications/VoiceXML

XHTML+Voice Profile 1.0 (W3C Note)
XHTML+Voice profile is designed for Web clients that support visual and spoken interaction.
http://www.w3.org/TR/xhtml+voice/
Top/Computers/Data_Formats/Markup_Languages/XML/Applications/VoiceXML

VoiceXML 2.0 Implementation Report
The VXML test suite provided by the W3C to demonstrate that the VXML standard can be implemented. This test suite is then used by the VoiceXML Forum as the basis for conformance activities.
http://www.w3.org/Voice/2004/vxml-ir/
Top/Computers/Data_Formats/Markup_Languages/XML/Applications/VoiceXML/Implementation_Platforms