Mufti Shaafi (r.a.) writes in the commentary of this verse that Allah Ta’ala has shown us three methods of Dhikr:
a) Dhikr Qalbi: The tongue doesn’t move but the heart and mind are involved in the remembrance of Allah
b) With the Dhikr of the heart, the tongue also engages in Dhikr but softly so that only the reciter hears. Both these meanings are incorporated in ‘And remember your Lord within yourself’.
c) Dhikr is made audibly but in a moderate tone. This method is taken from ‘without loudness’. Note: ‘without loudness’ negates Dhikr made in loud tones, not Dhikr in an audible manner.