Fix bug 2046705 where the output of the speech synthesizer is too low.

The language files for the SVOX Pico engine result in the output of
the synthesizer to be too soft, and barely audible on a phone speaker.
The change implements a low shelf filter on the output of the synthesis
and a drastic amplification. This works as intended because the
synthesized data contains too much energy in the lower frequencies that
is wasted on a phone speaker. Once filtered out, they leave room for
amplication to address the volume issue.