If you've been poring through the iPhone's settings app looking for control of Siri's volume, only to discover it isn't there, don't worry. The answer isn't that you can't adjust Siri's volume, you can. You just can't make Siri louder or quieter through the settings app.
To change Siri's volume, simply do the following:
1) activate Siri (press and hold the home button)
2) use the volume control buttons on the side of your iPhone