"Joy" is "gioia" in Italian. It comes from Latin gaudium. Gioia is feminine, while gaudium is neuter, though. It has to do with the fact that plural of gaudium is gaudia, that in the Middle Age was mistaken for the singular.
It is more uncertain how Latin bombus became Italian bomba, though. Here it can't be a false singular form, since the form bomba doesn't exist in Latin.