I experimented with ambient noises such as a waterfall, as you suggested, but I felt it was out of place with the increasing speed of the gameplay and didn't quite fit. I also tried searching for other types of audio, but I was having trouble finding fitting tracks in the short time left, so even though it feels bare, I prefered to leave it as is.
I might go back after the jam and work a bit more on the audio side, specially in some issues that appear in longer sessions.