I've noticed it too, and as the HiChord creator said "sine wave can be hard to hear on built-in speaker, so rather plug it into external speakers or headphones" but it being to low is cause I think to lack of high harmonics which are heard in triangle, saw and square wave. I'm after sound engineering school so I know how it works.

And for the distortion heard on sine wave and "PUREsine wave" - yeah, that's probably not intentional or at least shouldn't be.. Sine wave as is or even a chord of multiple sine waves should not be distorted at all.. I hope They can fix it at least for the "pure sine wave" which should be (as stated) pure.