Here are a few things to try:
# Turn it off, unplug it from the wall for at least a minute, and plug it in.
# You may be hearing the Intelligent Acoustic Control. To turn this function off, hold down "Metronome / Rhythm" and "Pianist Style", press the G#3 key to turn off (G3 to turn back on)
# You could also be hearing / missing the "Sound Boost" function. To turn this function on, hold down "Metronome / Rhythm" and "Pianist Style" and press the C4 key to turn on (C#4 to turn back off).
# Finally, you could try choosing a different "Voice". Try switching between Grand Piano, Bright Grand, Mellow Grand, and Stage Piano.