add a sound played when the host clicks "begin" in an mp game

src/game_config.cpp

@@ -152,7 +152,8 @@ namespace game_config
 		user_leave = "leave.wav",
 		game_user_arrive = "join.wav",
 		game_user_leave = "leave.wav",
-		party_full_bell = "bell.wav";
+		party_full_bell = "bell.wav",
+		mp_game_begins = "join.wav";
 
 		const std::string button_press = "button.wav",
 		checkbox_release = "checkbox.wav",

src/game_config.hpp

@@ -141,7 +141,8 @@ namespace game_config
 		extern const std::string turn_bell, timer_bell, receive_message,
 				receive_message_highlight, receive_message_friend,
 				receive_message_server, user_arrive, user_leave,
-				game_user_arrive, game_user_leave, party_full_bell;
+				game_user_arrive, game_user_leave, party_full_bell,
+				mp_game_begins;
 		extern const std::string button_press, checkbox_release, slider_adjust,
 				menu_expand, menu_contract, menu_select;
 	}

src/multiplayer_wait.cpp

@@ -25,6 +25,7 @@
 #include "mp_game_utils.hpp"
 #include "multiplayer_wait.hpp"
 #include "statistics.hpp"
+#include "sound.hpp"
 #include "wml_exception.hpp"
 #include "wml_separators.hpp"
 #include "formula_string_utils.hpp"
@@ -394,6 +395,7 @@ void wait::start_game()
 	}
 
 	LOG_NW << "starting game\n";
+	sound::play_UI_sound(game_config::sounds::mp_game_begins);
 }
 
 void wait::layout_children(const SDL_Rect& rect)