I really don't see the Light type happening.
If you're thinking of a Dark counter, that's already been established as Fighting.
If you're thinking of literal light, that's Electric's territory (see: Ampharos, Chinchou/Lantern)
If you're thinking of 'cutesy fairy things', those are generally Normal or Psychic types (but do fit in to other types).
Also, against the 'evidence' for a Sound type:
Yveltal looks to be based on a Wyvern, a creature known to use sound to damage in other games
A) Yveltal is clearly a bird. Not a dragon. B) I'm sure the Wyvern isn't known for using sound to damage any more than any other types of dragon.
Only Pokemon to make a sound in the trailer (other than Pikachu)
This is probably the most legitimate reason, but not really a reason at all - I'm sure they only put Yveltal's cry in the trailer for flavour. Try imagine it with no Yveltal cry, and it sounds more bland.
The game already recognizes sound based moves
And most of them are covered by Normal types - 4 out of the 18 moves covered by Soundproof are all different types (5 if counting Shadow Panic). And look at Exploud and Meloetta -both have influences from sound heavily in their designs (Meloetta's music bar hair, treble clef headpiece, quarter notes for arms and its eyes resembling whole notes. Don't need to explain Exploud) and they are both Normal types. Kricketune is the one exception I can think of that's got sound involved in it's design, but isn't a Normal type.
edit: added a bit/changed a bit around
If you're thinking of a Dark counter, that's already been established as Fighting.
If you're thinking of literal light, that's Electric's territory (see: Ampharos, Chinchou/Lantern)
If you're thinking of 'cutesy fairy things', those are generally Normal or Psychic types (but do fit in to other types).
Also, against the 'evidence' for a Sound type:
Yveltal looks to be based on a Wyvern, a creature known to use sound to damage in other games
A) Yveltal is clearly a bird. Not a dragon. B) I'm sure the Wyvern isn't known for using sound to damage any more than any other types of dragon.
Only Pokemon to make a sound in the trailer (other than Pikachu)
This is probably the most legitimate reason, but not really a reason at all - I'm sure they only put Yveltal's cry in the trailer for flavour. Try imagine it with no Yveltal cry, and it sounds more bland.
The game already recognizes sound based moves
And most of them are covered by Normal types - 4 out of the 18 moves covered by Soundproof are all different types (5 if counting Shadow Panic). And look at Exploud and Meloetta -both have influences from sound heavily in their designs (Meloetta's music bar hair, treble clef headpiece, quarter notes for arms and its eyes resembling whole notes. Don't need to explain Exploud) and they are both Normal types. Kricketune is the one exception I can think of that's got sound involved in it's design, but isn't a Normal type.
edit: added a bit/changed a bit around