Yep, that was it! I tested it today. Opened the Square App, plugged the reader into the headphone jack and waited for Square to recognize it, disconnected the reader, ringer volume goes to silent and locks in there until I restart the phone. I did this several times and it happened every time. I plugged a set of wired earbuds into the earphone jack, just to make sure it wasn't a Moto problem, and had no issues. Then I checked the App review on the Play Store, found another Moto X user with the same problem on both his/her 2014 & 2015 Moto X...just like me.