Is it obligatory to repeat saying "Alhamdulillah" after sneezing if those around me did not hear me, so that they can say "Yarhamukallah" to me?

It is not obligatory to repeat the praise (al-hamd) after sneezing, for praising Allah is recommendable (mustahabb) and not obligatory (wajib). It is not obligatory to vocalize it loudly, but the recommendation is achieved by raising one's voice with the praise so that those next to him can hear it. If those present do not hear him, then they are not required to say "Yarhamukallah" (may Allah have mercy on you), and it is sufficient that the angels say it if he praises Allah while he is alone.