I'm about to make your day, twice. 1: You can adjust the Siri app's volume the same way you can any other. Activate Siri, and then use the volume rocker button

I'm about to make your day, twice. 1: You can adjust the Siri app's volume the same way you can any other. Activate Siri, and then use the volume rocker button to adjust it up/down. 2: You can turn off voice responses altogether! In Settings>General>Siri, change "Voice Feedback" to "handsfree only". Now Siri will stay completely silent if you activate it with the home button ("Hey Siri" is hands-free, so you still get voice responses there).