Sour cream pound cake
If the package of cake mix got smaller (and otherwise didn't change) then you could try reducing every ingredient by a similar amount, to keep all the ratios the same. 

If the package decreased from 18 to 15 ounces, that's makes the new package 83.3% (15÷18) the size of the old one. You'll want to reduce other ingredients similarly. 

Your "18 oz recipe" was:


18 oz cake mix
4 eggs
½ cup sugar
1 cup oil
8 oz sour cream


With the eggs, an exact reduction would be 3⅓ eggs, and ⅓ egg is just silly. I'd suggest rounding down to 3. To keep the other measurements at round fractions, the ratios will get thrown off a bit.

If you keep all the measurements even fractions for a "15 oz recipe" you might try:


15 oz cake mix
3 eggs
⅓ cup sugar
¾ cup oil
6 oz sour cream


I've rounded everything down on that... Which means I think it might be a touch dry compared to the original.

A slightly fussier attempt at a "15 oz recipe" would be something like this:


15 oz cake mix
3 eggs
⅓ cup + 1 Tablespoon sugar
¾ cup + 1 Tablespoon oil
7 oz sour cream


That still might need some tweaking to get it "just right"... And of course, if Duncan Hines changed more than just the size of the box, your recipe might need more careful adjustment.