Ah. Stereo mix. That's what I couldn't figure out. Thanks xtermin8r.
As long as/once stereo mix has been enabled as described in the thread linked to above, a command along the lines of the following should work to change Winamp to linein input.
<?xml version="1.0" encoding="utf-16"?>
<!--VoxCommando 2.0.0.8-->
<command id="277" name="Enable TTS visualisation" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>Window.Focus</cmdType>
<params>
<param>winamp</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>InputKeys.Send</cmdType>
<params>
<param>{CONTROL+L}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>InputKeys.TextEntry</cmdType>
<params>
<param>linein://</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>InputKeys.Send</cmdType>
<params>
<param>{ENTER}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>VC.Pause</cmdType>
<params>
<param>1400</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>TTS.Speak</cmdType>
<params>
<param>one potato. two potato. three potato. four. testing milk drop visualisation.</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>Enable T.T.S. visualisation</phrase>
</command>