If VoiceAttack is running while you switch the Windows default recording device without using VoiceAttack, assuming the speech recognition is set up to use the Windows default recording device rather than a specific device (I.E. the "Windows Speech Recording Device" option on the "Recognition" tab of the VoiceAttack options window is set to use the entry beginning with "Default - "), you'll need to reset the speech recognition engine, which will restart it and configure it to use the new recording device.
This can be done using the "Reset Speech Recognition" option in the context menu for the main window's titlebar (which can be opened by clicking the VoiceAttack logo in the top-left corner of the window).
If you do use VoiceAttack to switch the default Windows recording device, E.G. using the "Change Default Audio Devices" action in a command, VoiceAttack will automatically restart the speech recognition engine for you (again, assuming the speech recognition engine is set up to use that device).