Sound-producing devices such as headphone sets or loudspeakers are disclosed that can removably mate with an external interface. The external interface may plug into a port on an exterior of the sound-producing device. The external interface includes conversion circuitry that is powered by a power source in the sound-producing device. The external interface receives a first audio-encoded signal from a source device such as a mobile phone, tablet computer, or the like, and converts such signal to a conditioned signal which is compatible with the sound-producing device. Connection between the source device and the external interface may be a first type of wired or wireless connection. The sound-producing device may operate with other source devices that produce different audio-encoded signals by replacing the external interface with a different external interface whose conversion circuitry converts the different audio-encoded signal to the same conditioned signal. Alternative sound-producing devices are also disclosed.