Xperia M2 Ring Volume Drops On my Sony Xperia M2, running Android 4.4.2,, two or three times a day, the sound volume of the ringtone and notification alert drops to a barely audible level.  When this happens, calls can be missed as the ringing may not be heard.

The volume setting does not change, just the volume itself.  If the phone is powered off and back on, then the volume is restored to normal.  No earphones / headset is connected to the phone at any time, either by pluggin in or by bluetooth.

It is possible that the volume drop occurs because the ringtone is being diverted to the internal, i.e. in-call, speaker.  

I am trying to work through all the installed apps to see if I can isolate one which is hijacking the sound but without success.  Is there a known issue here?  A known faulty app?
I believe I have tracked this to an app.

Three In Touch (version 3.4.5) is a pre-installed VOIP app from the service provider (Three) that uses Wi-Fi to route calls if there is limited signal.  After disabling this app the volume drop went away and re-enabling it has brought it back.

This is unfortunate as the app is otherwise very useful.