Glad tidings are for him who finds abundant invocations for forgiveness recorded in his book of deeds.
(Ibn Maaja)
When people close by can hear the recitation of the musalli in the silent Salaat (Zuhr and Asr), then Sajdah Sahw is necessary if the recitation was by error. And, if it was deliberately done, then the Salaat has to be repeated.How close by in metres?
If another person can hear the qiraat regardless of distance it will be classed as jahr (aloud) and sajdah sahw will be wajib