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.
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