Not a widget as such but you can get keyboard short cuts by adding
awful.key({}, "XF86AudioPlay", function () awful.util.spawn_with_shell("nyxmms2 toggle") end),
awful.key({}, "XF86AudioStop", function () awful.util.spawn_with_shell("nyxmms2 stop") end),
awful.key({}, "XF86AudioPrev", function () awful.util.spawn_with_shell("nyxmms2 prev") end),
awful.key({}, "XF86AudioNext", function () awful.util.spawn_with_shell("nyxmms2 next") end)
to your `rc.lua` file.
In addition, the official site widget has been updated and the code there now works fine for me.