In some ways, exterior silence is easier to maintain than interior silence.
We are sometimes able to refrain from speaking or giving our input when it isn't necessary. We can turn off the external noise.
However, maintaining "silence of the heart", involves stilling our internal noise, which is harder.
Exterior silence often reflects the interior silence. One can project exterior silence to the world, by reacting differently in front of different people in different circumstances. You can pretend to have interior silence by just not speaking or reacing.
However, it is hard to "fake", so to speak, internal silence.