The USB connection on the rear of the AXA25 is only designed for charging a device, such as a smart phone.
Please be aware that we do not advise that you use this port for charging a device and using it as a source at the same time, as this will create a ground loop that will cause interference during playback. This is due to the nature of the circuit and components used in the AXA25.
Please also note, this port is not an input so will not allow you to play audio directly to the unit.