Basically any upbeat song that reminds you to appreciate life in the middle of absorbing and applying theory. Anything from the Internationale to Red Army tunes to modern(ish) artists like Chumbawamba are allowed. Even Lofi.
Personal favorite of mine is Quand Fera-T-Il Jour, Camarade? by Mireille Mathieu. It's bittersweet, hopeful and resolute. Even if I don't speak French, I sometimes mumble to it.
Runner-up would be Solidarity Forever by Pete Seeger.
Lenin is always with you/Ленин всегда с тобой is a really beautiful song
El Pueblo Unido Jamás Será Vencido is another hit - written by Chilean socialists shortly after the coup of Allende
Utah Phillips "We Have Fed You All For A Thousand Years" live album is very joyous, lots of banter and fun songs from a real life IWW union organizer and anarchist. I don't agree with all his politics but I still really enjoy the album and listen to it a lot.