The title made me imagine the engine revving such that you'd hear different notes but... this is not a normal mower, it's a smart mower with a mobile app; computer with a blade. It could probably play MP3s for all I know.
What does this actually do?
https://www.youtube.com/watch?v=p0_RPlSe344
It's possible that's some kind of motor noise, but it doesn't sound like it to me.
EDIT: I realize when I say "motor noise" there's some ambiguity. I know there isn't a gas powered engine in this little mower. Revving an engine is exactly what I expected, and this isn't that. When I reference motor noise here, I just mean sound could be produced by a servo or similar, but I don't think that's what's happening.
The Renault F1 engine could famously play tunes this way.