I need to take an audio file recorded by a man and convert it to a child's voice and mix it with a background voice track.
I have searched the internet trying to find a good program to do this but I didn't find it. Is there a C# API that can help me to implement it myself?
I think using a API and writing it yourself may fall under the "too difficult to do so" category. I would recommend using a free multi-track audio editor like Audacity, it has a pitch shift ability (that will do your child voice requirement) and you can play two files on top of each other (to do the background voice requirement)
If you still want/need a API the key word to look for when searching for the child's voice ability is "Pitch shifting"