A poem or song that's written or performed in honor of a marriage is called a prothalamion. Your beautiful singing voice may get you invited to present a prothalamion at your cousin's wedding.
You can also call a celebratory marriage song a prothalamium, or one that's written especially for a bride an epithalamium. Whichever word you choose, it's fairly obscure and used in a purely literary context these days. Prothalamion comes from Greek roots: the "before" prefix pro-, and thalamos, "bridal chamber."