The Volume Anarchy
Ubuntu’s PulseAudio sound server (still used by default on many Ubuntu installations, even those that have transitioned to PipeWire with PulseAudio compatibility) has a controversial feature called “flat volumes.” When flat volumes are enabled, adjusting the volume slider for an individual application (such as Firefox playing a YouTube video) also moves the system-wide master volume slider to match. This means that turning up the volume in a single app can inadvertently crank your speakers to maximum, and a sudden notification sound from another application will blast at full volume because the master was silently raised. This behaviour is the opposite of what most users expect from per-application volume controls.
Disabling Flat Volumes in PulseAudio
To stop Ubuntu from automatically adjusting the master volume when you change an individual application’s volume, you must disable the flat-volumes feature in PulseAudio’s configuration.
Open a terminal (Ctrl + Alt + T). Edit the PulseAudio daemon configuration file: sudo nano /etc/pulse/daemon.conf. Scroll through the file and find the line that reads ; flat-volumes = yes (it is likely commented out with a semicolon). Change it to flat-volumes = no, ensuring you remove the leading semicolon to uncomment it. Press Ctrl + O to save and Ctrl + X to exit. Restart PulseAudio for the change to take effect: pulseaudio -k (this kills the current instance, and it will automatically restart). After this change, each application’s volume slider operates independently from the master volume. Raising Firefox’s volume to 100% will only affect Firefox; the master system volume will remain exactly where you set it. If your Ubuntu installation uses PipeWire instead of PulseAudio, flat volumes are typically disabled by default, but you can verify by checking /etc/pipewire/pipewire-pulse.conf for similar settings.