Isso se deve a PulseAudio configruation.
Como mencionado aqui link
PulseAudio supports per-application volume control, but by default this doesnt do much as you can only control these volumes from the pulseaudio volume control utility. Meaning that in an application like Audacious, when the output device is set to PulseAudio, and the volume control is set to hardware, it will adjust the master volume control, not the per-application volume control.
Portanto, para corrigir esse comportamento, configure flat-volumes = no in /etc/pulse/daemon.conf para ativar o controle de volume por aplicativo.