I have the book Muffins Galore, and I have made several of the recipes in the book, but some of my customers are complaining about the dryness of the muffins, I am baking a little lower in temp than the recipe calls for as I use dark pans and Texas sized muffin pans, can I substitute some of the liquid for sour cream?

I figured "yes" since most of the recipes for cake say that the slice is moist when sour cream is added.

Any other thoughts or suggestions?

Thanks in advance. 

I would be careful about substituting sour cream for milk in a muffin recipe.

First of all, the muffin recipes in that particular book need more liquid across the board.  Just plain adding 1/4-1/3 cup more of milk will solve the problem for a recipe with 2 cups flour.

You can then try using half milk and half buttermilk. Or 3/4 milk and 1/4 sour cream.