Windows – How to redirect sound to USB headset when plugged in

I don't know of any automatic solution, but here are several hotkey-using solutions you could look at :

How to change the default audio device with a hotkey
A AutoHotKey script to switch between audio sources

Soundswitch
Switch between headphones and speakers (have a look at version 3)

The thread Audio playback through headphones and speakers has some ideas that may be worth trying.

1. Open up alsamixer. Then press F6 to select your sound card, most likely ending with PCH.
2. Scroll right till you find the Auto-Mute option.
3. Press up or down arrow to disable it. Press Esc to quit alsamixer.

Use the following script to automate it.

#!/bin/sh

NAME=$(basename -- "$0")

command -v amixer >/dev/null 2>&1 || { echo >&2 "amixer not installed"; exit 1; }
command -v pacmd >/dev/null 2>&1 || { echo >&2 "pacmd not installed"; exit 1; }

amixer -c1 sset "Auto-Mute Mode" Disabled > /dev/null

if [ -z $1 ]; then
  echo -e "Usage:\nFront Speakers\t: $NAME 0\t\nHeadphones\t: $NAME 1\nBoth Speakers\t: $NAME 2"
  exit 1
elif [ $1 -eq 0 ] 2> /dev/null; then
  pacmd set-sink-port 1 analog-output-lineout
  amixer -c1 set Headphone 0% > /dev/null
  amixer -c1 set Front 100% > /dev/null
elif [ $1 -eq 1 ] 2> /dev/null; then
  pacmd set-sink-port 1 analog-output-headphones
  amixer -c1 set Front 0% > /dev/null
  amixer -c1 set Headphone 100% > /dev/null
elif [ $1 -eq 2 ] 2> /dev/null; then
  pacmd set-sink-port 1 analog-output-headphones
  amixer -c1 set Front 100% > /dev/null
  amixer -c1 set Headphone 100% > /dev/null
else
  echo -e "Invalid argument"
  exit 1
fi

This assumes that the card you are using is numbered 1. Also, it assumes that your PulseAudio ports are named analog-output-lineout and analog-output-headphones. Save it as something like speakers.sh and chmod to set execute bit.

Run speakers.sh 1 for Headphones and speakers.sh 0 for Speakers.