There's no genitive forms here. «Молоко» and «сок» are accusative case forms.
Genitive forms have partitive meaning, which can often be translated with 'some' (выпи́ть со́ка 'to drink some juice', вы́пить молока́ 'to drink some milk'; сок also has a 2nd genitive form, вы́пить со́ку 'to drink some juice'). However, we don't usually genitive it with Present tense forms.
This is because, 'some' is a quantity. And you can't measure quantity when something is in process. You either know the quantity of something you've alredy drunk (and then you use the past tense), or quantity of something you're planning to drink (and then you use the future tense), at least not with the verb 'пить'.
Accusative of сок and молоко is the same as nominative: сок, молоко́.
Why is there a soft sign ? Would "пю" be pronounced differently from "пью" ?