You may have a weak battery recharge your player with the AC Adapter/charger connected to the player and the power supply
Check if your headphone plug has been inserted into the headphone jack of the unit completely· When you press the PLAY button, make sure that it is pressed and held for long enough and the mode dial is at the MP3 playback mode
Check if the volume is at an audible level. Increase the volume using the VOLUME + button
If your camcorder is in pause mode, press the PLAY button to resume playback