Good to know, and it makes sense. For fun, when you have the time, could you test out the following possible alternatives and let us know the outcomes?
<?xml version="1.0" encoding="utf-16"?>
<!--VoxCommando 2.1.0.3-->
<commandGroup open="True" name="Different TTS to Sonos tests" enabled="True" prefix="" priority="0" requiredProcess="" description="">
<command id="141" name="Another method for sending TTS" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="This is supposedly an "outdated" function, but it seems like it might be useful. According to the documentation: "This action will pause the current playback, play the alert, and then the playback will be resumed."">
<action>
<cmdType>TTS.Speak2Wav</cmdType>
<params>
<param>This is the TTS announcement</param>
<param>C:\VoxCommando\tts.wav</param>
<param>False</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Vera.Raw</cmdType>
<params>
<param>data_request?id=action&DeviceNum=184&serviceId=urn:micasaverde-com:serviceId:Sonos1&action=Alert&URI=x-file-cifs://192.168.81.191/VoxCommando/tts.wav&Volume=50&Duration=15</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>Another method for sending TTS</phrase>
</command>
<command id="145" name="Test the Sonos plugin's built-in TTS" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="According to the plugin documentation "This action will pause the current playback, say the text, and then the playback will be resumed"">
<action>
<cmdType>Vera.Raw</cmdType>
<params>
<param>data_request?id=action&DeviceNum=184&serviceId=urn:micasaverde-com:serviceId:Sonos1&action=Say&Text=This is the TTS announcement&Language=en</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>Test the Sonos plugin's built-in TTS</phrase>
</command>
</commandGroup>