Thanks. Your fs2_open.ini file, which specifies your hardware config, shows that a playback device is selected under the file's "Sound" section, but the sound device setting that appears alongside the speech settings is blank. Odd. The selected speech voice might not be valid, but since it's just a number (index into your installed TTS voices), it's impossible to say.
Some things to try:
1. Go to Knossos's settings menu (gear icon in upper-right). Under "Audio" section, uncheck Enable EFX. Perhaps unrelated, but EFX isn't useful. It was added as part of the new sound code back in 2012 but AFAIK has never been used.
2. Also on Knossos's settings menu, under the Speech section, make sure that a Voice is selected in the drop-down box, and press the Test button. Do you hear a TTS voice?
3. Press the Save button on Knossos's settings menu before leaving the settings menu. The button is towards the top of the settings menu, on the left.
4. To rule out the cause being related to a mod, please try playing without mods: on Knossos's Home tab, choose "Retail FS2".
Let us know how that goes.