Comment 1 for bug 1329749

GNOME Shell 3.12 and GTK+ 3.12 Available In The Ubuntu 14.10 Utopic Unicorn Repositories
GTK+ is always changing (and breaking) things like this.

 In GTK+ 3.10, toolbar icons default to 24x24, while the volume button defaults to 18x18.
In GTK+ 3.12, there are curiously enough two separate changes that each would have solved the problem: [1] changed the default size for the volume button to 16x16, which prevents the scaling, and [2] switched to using symbolic icons, which are rendered as black and thus happen to match. Regardless of the GTK+ changes, [3] solves the problem by setting the volume icon size to 24x24 to match the rest of the buttons.

[1] https://git.gnome.org/browse/gtk+/commit/?id=b617b003ef9e3d42ecf17b42348f72f87bca78a2
[2] https://git.gnome.org/browse/gtk+/commit?id=afa9339e7106432af6622824d9c8c0a67bbf543c
[3] g_object_set (volume_button, "size", GTK_ICON_SIZE_LARGE_TOOLBAR, NULL);