Please make sure to increase the volume level first by moving your finger in clockwise direction on the outside ring of the volume wheel. Your music needs to play to allow adjustment of the volume level.
Depending on how you have connected your ONE you will also need to adjust the audio output settings. Please go to SETTING > PREFERENCES > AUDIO and turn ON
a) the DIGITAL output if you connect the ONE to your amplifier/receiver/DAC via S/PDIF coax
b) SPEAKER output if you connect speakers directly to the ONE
c) ANALOG if you connect the ONE via the 3.5mm-to-RCA cable to your amplifier/receiver.