sorry for the spam, but I might have just the right music for this Retro Jam on my music licensing website PlayOnLoop.com
Of course, each music loop comes with a short edit that can be used for free under CC Attribution license (just browse the website and look for the button "Free Short Version"), thus perfect for quick games for jams and prototypes.
8-bit (NES style)
8-bit (Commodore / SID style)
16-bit (Mega Drive / Genesis / FM style)
16-bit (Super Nintendo / SNES style)
Good luck with your retro masterpieces! :D