Soft drinks are not complimentary anywhere on the ship, including the dining rooms.
However, lemonade, fruit punch, juices, coffee, tea, milk, etc. are available at no charge on Lido Deck, in the dining rooms, and from room service. You can also get complimentary cappuccino and espresso in the dining room, but not at Lido buffet area.
You can purchase the 'soda card', and use it anywhere on the ship. You can also take your own soft drinks on board at embarkation. Carnival allows a reasonable amount to be brought onto the ship.