With fresh, whole milk.
“Sweet milk” just means fresh whole milk. The term dates from when it was common to churn your own butter, which meant the house had milk in various states of culture. There are actually quite a few recipes that call for sour milk, though these days, we generally use buttermilk. (Which is sort-of-sour milk, technically, but we’re really getting off-topic.)
From the box of D.E. from Maumee, Ohio.
Dumplings
1 cup sweet milk
1 tsp. salt
1 tsp. Crisco
1 tsp. baking powder
Enough flour to make soft dough–roll thin. Cut in squares rolled thin. (Cook covered.)