Why soft drink is a mixture but water is a compound?
Have you never made a drink from cordial?
You pour a bit of cordial, a strongly flavoured sugar syrup, into a glass, and fill the glass with water. You could also fill the glass with sparkling water to make a fizzy drink. Of course, this is how all fizzy drinks are made.
The base solvent is water, whose formula you know. The solutes are sugar (usually a lot of sugar), and the flavourings are from oranges, lemons, caramel, and a whole lot of other stuff. In fizzy drinks one of the solutes is carbon dioxide, with which a fizzy drink is supersaturated (this is why the drink fizzes).