How can I switch between different audio output hardware using the shell?

In this case the card is always the same. What is changing between a switch and another is the "card-profile".

So the solution which actually worked is:

pacmd set-card-profile <cardindex> <profilename>

In my case I found all the card profiles with:

pacmd list-cards

And after I can switch between monitor and laptop speakers with:

pacmd set-card-profile 0 output:hdmi-stereo

And:

pacmd set-card-profile 0 output:analog-stereo+input:analog-stereo

Where 0 is the index of the card:

pacmd list-cards
Welcome to PulseAudio! Use "help" for usage information.
>>> 1 card(s) available.
    index: 0
    name: <alsa_card.pci-0000_00_1b.0>

And finally, in order to make the switch faster, I set up two alias in my .bashrc file:

alias audio-hdmi='pacmd set-card-profile 0 output:hdmi-stereo+input:analog-stereo'
alias audio-laptop='pacmd set-card-profile 0 output:analog-stereo+input:analog-stereo'

This way I can switch between audio from the monitor or from the laptop (headphones) typing in the shell: audio-hdmi or audio-laptop

I wrote a small indicator applet that lets you switch the sound output. No shell script but maybe helpful to you or other readers.

https://github.com/lkettenb/sound-output-switcher

I created a very small script based on the previous ones, which not only switches the audio but also the video output. It uses the disper to switch between displays.

Here is the code:

#!/bin/bash

CURRENT_PROFILE=$(pacmd list-cards | grep "active profile" | cut -d ' ' -f 3-)

if [ "$CURRENT_PROFILE" = "<output:hdmi-stereo>" ]; then
        pacmd set-card-profile 0 "output:analog-stereo+input:analog-stereo"
        disper -s
else 
        pacmd set-card-profile 0 "output:hdmi-stereo"
        disper -S        
fi

For me it is especially useful since I don't like to clone the displays. I either use one or the other. You may need to adapt the audio profiles to your specific system.