An earbud includes a housing, a speaker mounted within the housing, a processor mounted within the housing, a user input surface on the housing, and a set of self-mixing interferometry (SMI) sensors mounted within the housing. The set of SMI sensors includes a first SMI sensor configured to emit a first beam of light, and a second SMI sensor configured to emit a second beam of light. The second beam of light passes through the user input surface about an axis that is non-perpendicular to the user input surface. The processor is configured to adjust a parameter of the speaker at least partly in response to a first SMI output of the first SMI sensor and a second SMI output of the second SMI sensor.