Several years ago, a sofer gave a class and said that personal prayers (prayers composed by myself) outside the synagogue must be said aloud. You can say them quietly, but the words must be audible. But in Plaut's "The Torah - A Modern Commentary" (Revised Edition) on p.300 he is commenting on Parashat Vayigash 45:16
The report was heard in Pharaoh’s palace: “Joseph’s brothers have arrived!” And this pleased Pharaoh and his courtiers,
and writes that,
The Hebrew word for "report" is written in a constricted fashion. For it is the voice that is small which is often heard more than any other. This also teaches us to pray silently (my emphasis), for G-d hears the prayer of the heart.
So is it a matter of preference or custom whether to pray aloud or silently outside the synagogue, or are there halachic arguments relating to this question?