The ZX120 will only produce sound when it is powered on. To ensure the ZX120 is powered on, first unplug it from any USB power sources. While it is unplugged, press and hold the colored strip beneath the power logo until the power LED lights up. This may take up to fives seconds.
Some audio source devices, particularly iPhones and iPads, can change their volume levels when a speaker or headset is plugged in. Double-check that the volume on your audio source device is turned up to a reasonable level after the ZX120 is connected. If the ZX120 is plugged into a computer, also make sure that the volume is turned up for the program that is being used (i.e. the volume slider on the YouTube page, Pandora page, or within Windows Media Player.)
It’s also possible there is an issue with the headset jack that the ZX120 is being plugged into. Please try plugging the ZX120’s built-in 3.5mm audio connector into a different audio source device and playing audio to confirm that the issue is with the ZX120 itself.