iOS - Lower volume before playing video
You can lower volume without playing the video.

from another answer:

The control center works (swipe up from bottom of screen and slide volume slider as desired).
In iOS 9 at least, you can also go Settings -&gt; Sounds and under the Ringer and Alerts section, turn off &quot;Change with Buttons&quot;, then use the volume buttons on the side of the phone to adjust the media volume. You can leave this option off to always have media volume control with the hardware buttons if you are OK with the ringer volume being just off or on at a preset level using the silent toggle switch on the side of the phone.