A hearing aid includes a speaker driver, an optical source secured directly to the speaker driver, an optical detector secured directly to the speaker driver, a first light guide extending outwardly from the optical source and in optical communication with the optical source, and a second light guide extending outwardly from the optical detector and in optical communication with the optical detector. The first light guide is configured to deliver light from the optical source into an ear region of the subject via a distal end thereof, and the second light guide is configured to collect light from the ear region via a distal end thereof and deliver collected light to the optical detector. The hearing aid may include at least one signal processor configured to process signals produced by the optical detector, and at least one of the following: an accelerometer, a humidity sensor, an altimeter, a temperature sensor.