Crystal Nova has a sound that plays during the cast animation of the spell. But this sound is currently directly bound to the default Crystal Nova cast animation. This means:

1. The sound does not play when having the Yulsaria's Glacier equipped (its custom animation does not have the sound attached)
2. The sound does not play when used by the Arcana model (Arcana CM model's animation does not have the sound attached)
3. The sound does not play when used by other heroes than Crystal Maiden (Rubick or Ability Draft)
4. The sound does play when Crystal Maiden casts her Q ability, no matter what her Q is (Ability Draft)

For the immortal and arcana, it gives CM an advantage over the default animation (no sound = one less clue for enemies that it is being cast = less likely to attempt to avoid it).

Ideally, the sound should be part of the ability code and fire whenever the spell enters cast animation (abilityphasebegin), and remove it from the animation itself. If you want to keep it on CM only, then add the sound to her other Crystal Nova animations as well.