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 “Thus, when you give to the needy, sound no trumpet before you, as the hypocrites do in the synagogues and in the streets, that they may be praised by others. Truly, I say to you, they have received their reward.