While adjusting the volume on your iPhone, iPad, or iPod touch is pretty obvious, you may not have known that Siri actually has its own, separate volume setting. This means that you can actually adjust Siri’s volume independently of the system volume, or the volume level used in other apps.
To do this, simply start up Siri by holding the Home button and then use the standard volume up and down buttons to adjust the volume. The standard volume adjustment overlay will appear, but you may notice that the number of bars shown will be different from whatever you’ve set on your iPhone home screen. Siri will remember this volume setting for you, allowing it to always respond at a fixed volume, regardless of wherever the system volume is set.