A childhood belief about songs

As a kid I'd never heard the word "accord" before when I first heard it in the last verse of "The First Noel", in the lines:

Now let us all with one accord
Sing praises to our heavenly Lord.

The only guess I could then come up with for the meaning of "accord" was that it might be short for "accordion". I thought that a neat image, singing Christmas songs with accordion accompanyment. But I puzzled as to why it said "one accord", as if something would be wrong with Christmas singing to the accompanyment of more than one accordion. Perhaps accordions are loud enough that two would drown out the singing, I thought.