Do you have the "Set Windows Default Multimedia Audio Playback Device on Startup" option on the "Audio" tab of the VoiceAttack options window enabled?
If not, is the "Windows Speech Recording Device" option on the "Recognition" tab of the VoiceAttack options window set to something other than the entry beginning with the "Default - " prefix?
VoiceAttack is being instructed to switch to a specific device, but that device is no longer available under the device ID it was previously (that ID is set by Windows when the device is installed or reinstalled).
Check the "Set Windows Default Multimedia Audio Playback Device on Startup" dropdown, if that is indeed what you're using, for a second entry with the same device name, and choose that one instead.
The same may apply to the "Windows Speech Recording Device" option.