It takes several seconds to turn down the volume. It takes one second to steal a phone. Turning down the volume has nothing to do with securing a phone from being stolen. In fact all the thief has to do is turn it off.
Click on this link for general information about what to do "If your iPhone, iPad, or iPod touch is lost or stolen" --> https://support.apple.com/HT201472
- The Find My service is the only recovery feature Apple offers. It MUST be set up before the device is lost. If you did not do that it is too late to do it now and there is no other feature Apple offers that can help you find it.