That’s a great question…
For the most part, for smaller games, a developer will find it easier/more affordable to contract someone who can just “take care of the game’s audio.” So that means music, sound design and likely taking care of VO needs.
Of course, every developer is different, but that’s the tendency.
As games (and their budgets) get larger, they are more likely to specialize their audio team, with separate composer(s), sound designer(s), etc.
This was one of the topics covered in the GameSoundCon Survey from a few months ago.