move the "ready to start" sound to the "turn bell" channel

This allows us to play all the sounds correctly in mp without
needing more than two UI channels.

The second UI channel was created in this commit:
e39797b

Together these fix a problem reported here:
http://forums.wesnoth.org/viewtopic.php?f=4&t=41059#p576534

src/multiplayer_connect.cpp

@@ -560,7 +560,9 @@ void connect::process_network_data(const config& data,
 	update_playerlist_state(result.second); //result.second is the silent flag
 	if (!was_able_to_start && engine_.can_start_game()) {
 		DBG_MP << "play party full sound" << std::endl;
-		sound::play_UI_sound(game_config::sounds::party_full_bell);
+		if(preferences::UI_sound_on()) {
+			sound::play_bell(game_config::sounds::party_full_bell); //this is play_bell instead of play_UI_sound to economize on sound channels. UI only has two sounds, and turn bell has a dedicated channel.
+		}
 		desktop::notifications::send(_("Wesnoth"), _ ("Ready to start!"), desktop::notifications::OTHER);
 	}
 }