/usr/share/gnome-speech/sources/swift-synthesis-driver is in gnome-speech-swift 1:0.4.25-5ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | #!/bin/sh
# Run swift via padsp if available.
if pidof pulseaudio > /dev/null && which padsp > /dev/null ; then
/usr/bin/padsp /usr/bin/swift-synthesis-driver.bin $@
else
/usr/bin/swift-synthesis-driver.bin $@
fi
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