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.
A spectrogram is a time-varying spectral representation that shows how the spectral density of a signal varies with time. Also known as spectral waterfalls, sonograms, voiceprints, or voicegrams, spectrograms are used to identify phonetic sounds, to analyse the cries of animals; they were also used in many other fields including music, sonar/radar, speech processing, seismology, etc
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:
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation. In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph of the varying quantity against time or distance
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.
A common word I hear used a lot in music production is Waveform.
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.
Can it be called a graph? As I understand it, it's a temporal graph, whilst a spectral graph shows frequency along the X axis. Aren't they two different ways of looking at a waveform? If you can call the basic sine wave a graph (and you can, can't you?), why can't you call the waveform depiction a graph? (My lecturer says it's not a graph perse (sic), but there's lots of arguments to support that it is a 'graph'). It's got two axes. It's on a co-ordinate plane. It represents data or values in an organized manner. The points on it represent the relationship between two or more things. It's a graph!