Design and operation techniques for an earphone-based game controller and health monitor are described herein. In one example, two “ear bud” style earphones are configured as I/O devices, each including a speaker to output sound and an accelerometer to receive an input of motion imparted by a user. The I/O devices may be connected to a mobile device, such as a cellular phone. Accordingly, the I/O devices may be inserted into a user's ears for listening or may be used as game controllers, such as by moving one I/O device in each hand. In a further example, a thermometer may be used in at least one of the I/O devices, and may gather health data by measuring the temperature of the user. And in a further example, a microphone may be used in at least one I/O device to gather heart rate and respiration-quality data regarding the user.