Athreos triggers when any of its controller's creatures die, and no matter what they look like in the graveyard, they can still be returned unless 3 life is paid.
If Mutavault is a creature when it dies, it will trigger Athreos and will be returned to your hand unless the opponent pays 3 life.
Athreos's trigger gets a little tricky when combined with triggers that try to return the card to the battlefield!
Athreos can't return tokens that die, but the opponent can still choose to pay 3 life for no reason! Also, if a card doesn't actually hit the graveyard, Athreos can't trigger (e.g., with Rest in Peace).