For anyone stumbling upon this topic: third-party TTS voices are likely to support SSML, however the SSML specification dictates that you use the "xml:lang" tag to specify which language the text will be read out in.
This means that if you're using a US English voice, you'd want to use a header like
<speak version="1.0" xml:lang="en-US">
but if you're using a UK English voice, you'd want to use a header like
<speak version="1.0" xml:lang="en-UK">
If you, for example, set "xml:lang" to "en-US", if the voice you have selected in the "Voice" dropdown is not a US English voice, an available US English voice will be used instead of the selected voice.
This is not VoiceAttack-specific.