I see currently a Chord is the only thing implementing the Playable trait. Does it fit your vision for the crate for Note and Pitch to implement Playable? In theory, it could be possible to do

impl<F: HasFrequency> Playable for F { ... }

However I think that clashes with the existing implementation of Playable for Chord - I could not get it to work correctly for some reason.

P.S. more of a breaking change, and just a suggestion: in Playable::play, I think it might be more practical to take std::time::Duration instead of f32 as arguments. When I first used this function I needed to dig at the source code/try out empirically to find out what the unit of length and delay and fade_in is.