The aim of Mixed Reality (MR) technology is to overlay the physical reality with virtual stimuli indistinguishable from real stimuli. In the visual domains, the main technical paradigms of MR are see-through cameras or transparent glasses. Analogously, in the auditory domain, the two competing models are hear-through and transparent headphones. In this work, we evaluate the fidelity and transparency of multiple commercial and experimental headphones. Further research directions include the perceptual influence of headphones for listening to real as well as virtual sound sources.