There are different stages of simran. In the initial stages we should practise simran by speaking it loudly. A mala can be used to help us concentrate.
At some point with Guru jis kirpa, a stage is reached where the lips cease to move and mental simran continues. This stage induces mental peace and joy. As we grow spiritually, mental simran will always or whenever we choose progress unceasingly. The physical aspect of simran ends and mental simran continues from day to day.
Read bandginama by Raghbir Singh Bir for further clarification.