hitsound was accidently mandatory, but it is not.

source/glest_game/types/skill_type.cpp

@@ -967,6 +967,7 @@ void AttackSkillType::load(const XmlNode *sn, const XmlNode *attackBoostsNode,
 			throw megaglest_runtime_error("Damages percentages of projectiles don't sum up to 100 %");
 		}
 
+		if(sn->hasChild("hitsound")==true){
 			//general hit sounds, individual ones can be set in projectiles
 			const XmlNode *soundNode= sn->getChild("hitsound");
 			if(soundNode->getAttribute("enabled")->getBoolValue()){
@@ -984,6 +985,7 @@ void AttackSkillType::load(const XmlNode *sn, const XmlNode *attackBoostsNode,
 				}
 			}
 		}
+	}
 
 	//splash
 	const XmlNode *splashNode= sn->getChild("splash");