Electronic devices may be provided with audio circuits and circuitry configured to support communications and test mode operations. During normal operation, a connector such as an audio connector may be inserted into a connector port in an electronic device. The audio connector may be associated with a headset or other accessory and may be used to carry audio signals. During test mode operations, a tester may be coupled to the connector port using an audio connector. The tester may generate voltages, resistances, time-varying signals, or other input that directs the device to configure switching circuitry to support testing. Monitoring circuitry in the device may be used to detect input from the tester. In response to detected input from the tester, the switching circuitry may be adjusted to couple a control circuit that supports test mode operations to the audio connector.