Ah, a plethora of good selections! (And some silly ones.)
If you’re going to sing the Internationale, avoid that crusty old British version. It doesn’t even come close to translating the original French. Here’s a better version:
Arise, ye prisoners of starvation,
Arise, ye wretched of the earth -
For justice thunders condemnation
A better world’s in birth!
No more tradition’s chains shall bind us;
Arise ye slaves, no more in thrall!
The earth shall rise on new foundations -
We have been naught, we shall be all.
'Tis the final conflict
Let each stand in his place
The international working class
Shall free the human race (2x)
Here is a piano score you can print out.
Let’s see, what else? Definitely “Solidarity Forever”; the link Happy Lendervedder provided is a great one, although I’d recommend adding this as the second verse:
It is we who plowed the prairies, built the cities where they trade;
Dug the mines and built the workshops, endless miles of railroad laid.
Yet we stand outcast and starving 'midst the wonders we have made -
But the union makes us strong!
[Chorus].
Hmm, what else? Oh yes, definitely ‘Bandiera Rossa’. This site may take a bit to load but it’s got the music and the words set to it. (Although I substitute the word “socialismo” for “comunismo” in the last line.)
To piss your boss off even more, sing There is Power in a Union, to the tune of Battle Cry of Freedom.
And, finally, a song about the fight which got us May Day in the first place - and remember, folks, May Day started right here in the US, not in Europe or revolutionary Russia - The Eight Hour Song.