Minecraft isn't built for flying mobs; the Bat took months of development.Additionally, leashes do not stretch otherwise they would be useless for tying mobs to posts.
Are you using a resource pack? They can substitute sound.
https://mojang.atlassian.net/browse/MC-23017Same here, I think it's got a problem with the new resource pack format.
Minecraft isn't built for flying mobs; the Bat took months of development.
Additionally, leashes do not stretch otherwise they would be useless for tying mobs to posts.