One answer can be the one given by @Koen. But there is another sound settings app, called Pulse Audio Volume Control. It must be with the installation. If it isn't there, install it using sudo apt install pavucontrol. Here is a screenshot here on my Xubuntu system:
search "sound" in "app search"
click on super key (= windows key on windows keyboard) to get to app search
select "Sound" section of settings app
One answer can be the one given by @Koen. But there is another sound settings app, called Pulse Audio Volume Control. It must be with the installation. If it isn't there, install it using
sudo apt install pavucontrol
. Here is a screenshot here on my Xubuntu system: