Trumpets may have been used to call assemblies together for prayer in the synagogue.1 It is unlikely that they were blown when someone gave money, but the image/idea is used by Jesus as a metaphor for drawing attention to one’s pious act of giving.2 It may be that those who gave large contributions in the synagogue were recognized in some way. Whatever the case, alms giving has historically been used as a means to get a reputation for generosity and piety.3
2 Donc, lorsque tu fais l'aumône, ne sonne pas de la trompette devant toi, comme font les hypocrites dans les synagogues et dans les rues, afin d'être glorifiés par les hommes. Je vous le dis en vérité, ils ont leur récompense.