In all places and at all seasons you may sing with the mind. But whether you walk in the marketplace, or begin a journey, or sit down with your friends you may rouse up your mind or call out silently.

 So also Moses called out, and God heard him. If you are an artisan, you may sing sitting and working in your shop. If you are a soldier, or if you sit in judgement, you may do the very same. One may also sing without the voice, the mind resounding inwardly. For we sing, not to men, but to God, who can hear our hearts and enter into the silences of our minds.
Saint John Chrysostom