speak with dead, what is a mouth

Speak with dead requires the corpse has a mouth. i usually take pact of the tome and speak with animals is a level 1 ritual. So it made me think. What is a mouth.  Is a beak a mouth for example.  What animal um mouths are mouths. i don't know what the general term is if its not mouth. 

But im seeing a potential synergy here, just not sure how big. its not like dead animals are everywhere you go, but it does expand the range a bit.

i guess the other question is corpse a specific game term that i can't find that only applies to humanoids or something. 

edit to add does it even work as if its a corpse is it no longer a beast so speak with animals would do nothing

A mouth is a mouth.  Yes, a bird's beak is around it's mouth.  

The creature speaks whatever 'language' it knew when it was alive.  So I'd say Speak with Animals should work, since it would 'speak' how it 'spoke' as an animal. 

(Technically, if an animal knew a real language, even though it couldn't speak it in life, it's animating spirit would know it, and could presumably speak it? Clearly biological capability isn't required, since many corpses that are legal targets no longer have a larynx, and the spell doesn't require one.)

This gets weird if the creature knew a sign language, as then you should need hands and fingers, not a mouth... I don't think the writers ever considered a non-spoken language...

My understanding is, a corpse is an object, not a creature. So it would not be a beast, and speak with animals wouldn’t work. I believe there is some debate about that corpse-as-object definition. 
Personally, I’d probably allow it, anyway. Seems like it could be fun. But that doesn’t mean the animal will be very helpful. But animals in my world rarely pay attention to much beyond food, predators and reproduction. And have almost no concept of time.