IF THIS (Device is plugged in), THEN THAT (Mute ringtone)

Trigger Channel: Android Battery Device is plugged in: This Trigger fires every time your Android device is plugged in. Action Channel: Android Device Mute ringtone: This Action will mute your Android device's ringtone. If your device is already muted, it will remain muted. If device is plugged in, then mute ringtone.