Soda is free at table service meals (i.e. if you eat breakfast and lunch at Lumiere's, which is available for free, you can ask for sodas and they're free). During the evening meal, sodas are free in the main dining rooms.
DCL offers a program for unlimited soda. It's $5 per day, and you get a sticker on your KTTW card. For a single $3 fee you also get a souvenier mug. If you booked with
AAA, they sometimes offer the sodas as a booking perk.
Sodas ordered from room service have a fee. Room service food including milk, tea, and coffee are included.
Alcohol is always charged except at the captain's gala (usually semi-formal night I think), and at the
Castaway Club (repeat cruisers) reception.
Hope this answers your questions. Welcome to DIS!