Ahhh good point! Honestly, we did plan on a number of features (the water was going to be a place you could jump into and affect the music) that didn't make the "free game" cut.
One thought we had re: walking SFX is that it sort of interrupts the music - hence why there's no notification SFX when you activate stuff. At one point we were playing the game and playing instruments along with it, so any extra noises weren't great when recording.
We just hit 2,000 downloads though and were planning on doing an update, so we'll put the walking sfx in there as well!