As in, your browser's sound output is muted while VoiceAttack is running? Is this change reflected in the volume mixer (hold the left Alt key and click the wrench icon on VoiceAttack's main window)?
Does this also occur if VoiceAttack is not running, but you start the Windows Speech Recognition application?
Are you running any plugins or third-party profiles? VoiceAttack does not normally mute any sound devices or applications without being explicitly set up to do so.