I occasionally have the need to capture the audio that is given out to the speakers on Ubuntu.
The following little script has proven to work well across my devices and regularly does me a good turn, so I thought I'd share it with you, either to use directly or to act as a reference about capturing sound / audio with pulseaudio.

File: pacapture.sh

#!/bin/bash

if ! which sox pacmd parec pactl amixer >/dev/null; then
    echo "Need to install some packages"
    sudo apt install sox alsa-utils pulseaudio-utils
fi

OUTPUT_FILE="${HOME}/Music/capture/$(date '+%Y-%m-%d-%H-%M-%S').wav"
mkdir -p "${HOME}/Music/capture"

# Get sink monitor:
MONITOR=$(pactl list sinks | grep -o -m 1 'alsa_output\..*\-stereo\.monitor$')

echo "Recording from ${MONITOR} to ${OUTPUT_FILE} ..."
echo "set-source-mute ${MONITOR} false" | pacmd >/dev/null

amixer -q -D pulse sset Master 1%

# Record it raw, and convert to a wav
parec -d "${MONITOR}" | sox -t raw -e signed -r 44100 -L -b 16 -c 2 - "${OUTPUT_FILE}"

Especially notice the line

amixer -q -D pulse sset Master 1%

To actually capture sound, the output to the speaker mustn't be muted. This is important, so while you're capturing sound, never turn the volume to zero. Hence I set the volume to 1%, which is pretty much like muted on my systems (meaning it's so quiet, I don't even hear it). Also note, that I don't have multiple audio devices. If you have multiple devices, you might have to run pactl list sinks and adapt the grep pattern to fit your particular environment.