It looks like flakes of a material with higher melting point and cohesion value than the base plastic would be injected together.
Alternatively small beads could be splattered onto the mold surface, or it could be In-Mold-Labeling but those methods would be more labor-intensive. Also looking at different Bose Freestyle earbuds which have the same effect applied to them, they appear to all be different so I think it’s a simple mixing of different plastic flakes into the IM machine. It’s a cool effect so if anyone knows a manufacturer that does this, that would be great!