Interfacing the voice and visual components of X+V documents is accomplished through a combination of ECMAScript, JavaScript, and XML Events.
Pitch, volume, and other characteristics can be customized using CSS and Speech Synthesis Markup Language (SSML) however the Opera web browser doesn't currently support all these features.
Here is some PHP code that returns "true" if and only if the requesting browser supports XHTML+Voice: Speech Application Language Tags (SALT) is a very similar format developed by Microsoft in 2001 to compete with VoiceXML and XHTML+Voice.
SALT also provides users with multimodal support including grammar based recognition and speech synthesized output.
Many different companies support VoiceXML and XHTML+Voice by providing various development tools and in particular IBM and Opera Software.