# Create a new "Output Device" to receive the transformed sound:
# https://askubuntu.com/questions/421947/is-there-a-way-to-modulate-my-voice-on-the-fly
pactl load-module module-null-sink

# Optional, but useful:
# https://askubuntu.com/questions/18958/realtime-noise-removal-with-pulseaudio
pactl load-module module-echo-cancel

# Create a new "Recording" stream with various voice effects on it:
# http://manpages.ubuntu.com/manpages/xenial/man1/sox.1.html
sox -t pulseaudio default -t pulseaudio null rate -v riaa pitch 250 dither

# On the "Recording" tab:
# - Make sure "Echo-Cancel Source Stream from" is set to use the input device.
# - Make sure "SoX: record from" is set to use the echo-cancelled input.
# - Make sure (target program name here) is set to "Monitor of Null Output".