It's used by several packages as a local flag, and its meaning seems to be similar enough.
app-editors/emacs:sound - Enable sound app-editors/emacs-vcs:sound - Enable sound app-misc/anki:sound - Enable support for adding sound to cards games-arcade/tuxanci:sound - Enable sound games-board/pysolfc:sound - Enable sound support usingdev-python/pygame games-roguelike/angband:sound - Enable and install sounds games-rpg/eternal-lands-data:sound - Adds in-game sound effects. games-strategy/freeciv:sound - Add support for sound provided by media-libs/sdl-mixer gnome-extra/gnome-games:sound - Enable sound using media-libs/libcanberra media-libs/libcanberra:sound - Install x11-themes/sound-theme-freedesktop to get sounds on Gnome and Xfce. net-irc/xchat-gnome:sound - Enable sound event support with media-libs/libcanberra net-p2p/transmission:sound - Enable sound event support with media-libs/libcanberra xfce-base/xfce4-settings:sound - Enable sound event support with media-libs/libcanberra
