Sounds and graphics were typically created in code back in those days too, rather than using external image and sound files which is the common practice today.
Back then, we had very limited memory to work with, so coding the graphics and sounds are a great way to keep your file sizes very low.
Of course, that is taking it to the extreme, but it is doable if you have the time.
My entry is an example of that although is was previously published, hence the quick submission.
With a month for this Jam, you might want to explore that option for a great learning experience if you have never tried it.