What is this called? How do we say oscillogram of the sound in one word?
The visual display of a sound wave can take many forms. You ask what the "graphical representation of a sound" is called, and you show an example. There are different terms for different forms of graphical representations of sound, for example sonogram, (audio)spectrogram, waveform, (audio) oscillogram. Which one to use is a technical matter.
This image shows Time (X axis) vs. Frequency (Y axis).
However, the picture you show in your question is different, called an (audio) oscillogram or a waveform:
This image, like yours, shows Time (X axis) vs. Amplitude (Y axis) and is generally called a waveform or oscillogram. Use "audio" if you want to refer specifically to the waveform of a sound as opposed to some other signal.
EDITED thanks to input from commenters.
Since you need a word that would indicate sound, then it would be as simple as "audio waveform", since plain waveform could indicate any kind of waves.
I would also like to point out that a sonogram is not the same as a waveform, since sonogram in two dimensions represents intensity vs time and in three dimensions intensity vs frequency vs time.