Lead by example
As adults, we, too, cannot help but talk in loud voices whenever we feel excited and happy. If your children are feeling those emotions, it would be unfair to restrict them when they have heard you do the same. Unless you are having shouting matches, make it a point that you (as an adult) speak in an indoor voice when inside the house. Then, if, you have never talked loudly and consequently they have never heard you talk loudly, you can easily trick children to talk in an indoor voice.