The missing icon is called multimedia-volume-control. Installing hicolor-icon-theme should help.

(In reply to Julian Sikorski from comment #1)
> The missing icon is called multimedia-volume-control.

thanks; so, currently it is provided by these packages:

adwaita-icon-theme-3.14.1-1.fc21.noarch : Adwaita icon theme

control-center-1:3.14.2-2.fc21.x86_64 : Utilities to configure the GNOME desktop

echo-icon-theme-0.3.89.0-0.19.20081003gitcc6da5b.fc21.noarch : Echo icon theme

elementary-xfce-icon-theme-0.4-2.fc21.noarch : Icons for Xfce based on the elementary Project Icon Theme

gnome-icon-theme-3.12.0-2.fc21.noarch : GNOME icon theme

gnome-themes-standard-3.14.2.2-1.fc21.x86_64 : Standard themes for GNOME applications

kfaenza-icon-theme-0.8.9-6.fc21.noarch : Faenza-Cupertino Icon Theme for KDE

kmenu-gnome-0.8-9.fc21.noarch : K Menu with Gnome directory

mate-icon-theme-1.8.0-2.fc21.noarch : Icon theme for MATE Desktop

mate-icon-theme-faenza-1.8.0-2.fc21.noarch : Extra set of icon themes for MATE Desktop

mate-themes-1.9.3-0.3.git20141226.962f516.fc21.noarch : MATE Desktop themes

mingw32-adwaita-icon-theme-3.14.1-1.fc21.noarch : MinGW Adwaita icon theme for MingGW

mingw64-adwaita-icon-theme-3.14.1-1.fc21.noarch : MinGW Adwaita icon theme for MingGW

oxygen-icon-theme-4.14.3-1.fc21.noarch : Oxygen icon theme

oxygen-icon-theme-4.14.3-1.fc21.noarch : Oxygen icon theme

rodent-icon-theme-5.0-4.fc21.noarch : SVG scalable icon theme by Rodent


however, they don't seem to provide any virtual 'desktop-icon-theme' like alias that could be depended upon without forcing any particular package, so basically this cannot be solved in pavucontrol

so ... it could be proposed to add such virtual provides, but in that case, it'd be nice to have a list of icon names that must be present so that applications depending on it can rely on them

but I'm no expert in this field :-( ... is this the list that should be used as normative? - http://standards.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html#names

> Installing hicolor-icon-theme should help.

actually, hicolor-icon-theme doesn't provide any icons and doesn't require any package that would provide them

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