Sound levels are governed by two things: the low-level ALSA driver and the high-level PulseAudio controls. So adjust the levels in ALSAMixer and also PulseAudio Volume Control. I don't use GNOME these days, so have a look for the latter in one of the menus. If you can't find it, just open a Terminal window and enter the command pavucontrol
, which should launch the PulseAudio Volume Control GUI. Regarding the icon, as I don't use GNOME I'm not sure, but it'll be something like right-clicking on the Panel and selecting something.
Low-level (ALSA) sound driver volume control via ALSAMixer:
High-level (PulseAudio) sound control via PulseAudio Volume Control app:
High-level (PulseAudio) volume control via PulseAudio Mixer (KDE) in the Panel: