scenes/levels: use contained assets manager for musics

imperium-porcorum.love/scenes/levels/init.lua

@@ -53,7 +53,7 @@ function Level:loadMission(levelID, missionID)
 
   self.world = World(self, self.datas.missions.map)
   self.world:load()
-  assets:setMusic(self.datas.missions.music)
+  self.assets:setMusic("assets/music/" .. self.datas.missions.music)
 
   self:launchMission()
 end
@@ -113,8 +113,8 @@ end
 function Level:launchMission()
   self.playermanager = PlayerManager(self)
   --self:resetSpawnAndEntities()
-  assets:silence()
-  assets:playMusic()
+  self.assets:silence()
+  self.assets:playMusic()
 
   self.playermanager:addPlayer(1)
   self.playermanager:spawnPlayer(1)