freeswitch/conf/insideout/lang/en/en.xml

<include>
  <language name="en" sound-path="$${sound_prefix}" tts-engine="cepstral" tts-voice="callie">
    <X-PRE-PROCESS cmd="include" data="demo/*.xml"/> <!-- Note: this now grabs whole subdir, previously grabbed only demo.xml -->
    <!--voicemail_en_tts is purely implemented with tts, we have the files based one that is the default. -->
    <X-PRE-PROCESS cmd="include" data="vm/sounds.xml"/>  <!-- vm/tts.xml if you want to use tts and have cepstral -->
  </language>
</include>