You can use a hack to change Siri to Jarvis. However, it isn't part of regular Siri voices. Follow
these steps:
Open the Cydia and add http://repo.cydro.us in the available sources. Then, click on Sources > Edit > Add to add it.
After adding the repo, go to the changes tab and select the Jarvis for Siri package.
Now respring your iPhone, and launch Siri. It will start sounding like Jarvis.