Otoacoustic emissions (OAE) are sounds generated when the outer hair cells move in a healthy cochlea and provide information about their function. OAE testing is commonly used as part of universal infant hearing screening protocols in high-income countries. OAE equipment however is expensive hindering early hearing screening in developing countries that bear the disproportionate brunt of disabling hearing loss. Here, we report the design and clinical testing of a low-cost OAE probe using off-the-shelf earphones and microphones, with a material cost of $10. Our design sends two pure tones through each of the headphone's earbuds and records the OAEs generated by the cochlea using a microphone. By running algorithms on a smartphone connected to the earphones using its headphone jack, we can detect distortion-product OAEs. We conducted a clinical study on 201 pediatric ears at otolaryngology and hearing clinics and achieved a sensitivity of 100% and a specificity of 88.9% for screening positively for hearing loss, which was comparable to a commercial OAE device. Given the prevalence of inexpensive earphones and budget smartphones, our low-cost tool could potentially help increase access to early detection of hearing loss, particularly in developing countries.