TL:DR at bottom!
I sifted through the hardware code of Makecode arcade and found how it runs sound effects! I was hoping to find some sort of sound file player, but instead they simply play a bunch of sine waves just as we can in the coding menu. Although they do have some extra shapes beyond just sine, square, sawtooth, and noise! That’s how they make the other stock noises in Makecode arcade that we cant, like the other notes in the melody player and the sound effects.
I also found that for the different curve shapes of the sine waves (Linear, curve, and logarithmic) the program seems to step along 5 ms intervals, calculating what the frequency output should be. This holds A LOT of promise for sine waves in Makecode to be functions. (Not code functions, I mean like the polynomial ones, like 3x^2+2x-4!) Speech can be converted into sine waves, which can be played as functions!! Oh, and every other sound, of course. (Take this with a bunch of salt as my JS knowledge is so old, I could very well be misinterpreting this!)
So… If the mods wanted to they could try and change their two-point slope system into a function one for the sine wave player… 
TL;DR: Makecode arcade has actually been playing other sine wave shapes this whole time. Also, using functions for sine waves in Makecode Arcade is 100% possible!
(An image for reference of where I found the second bit)
(Oh yeah, and this is assuming you can’t edit the makecode hardware shown here. If you can, tell me!)