Part of Siri's charm for me is her British accent that lends an air of assurance to her answers. Apple's assistant, however, can be a bit too forward at times, inserting herself into conversations when she has not been addressed. There is a setting that lets you silence Siri without disabling the feature. With this setting, you can toggle Siri's voice feedback on and off using the lock switch.
On an iPhone, head to Settings > General > Siri > Voice Feedback.
Choose Control with Ring Switch.
Now, you can flip the lock switch (aka ring switch) to mute Siri. You'll still be able to ask Siri questions, but the answers will appear on screen without the accompanying audio.
On an iPad, you will need to have the lock switch (dubbed the side switch in this instance) set as Mute and not Lock Rotation in order to use it to mute Siri. The side switch setting can be found on the General page in Settings.
(Via OS X Daily)