Playing Audio from SD - Get VU, or audio waveform
You are sampling your audio signal with only 20Hz. That is very low frequency for an audio signal, that humans can hear. Generally you need to sample your signal with double of the highest containing frequency at minimum, or you will only get nonsense (you can google Nyquist Rate for more information). Also a Nano does not have a real DAC, so the audio is generated via a PWM signal, which turn the line on and off very fast, so you only see the waveform only, if you apply a low pass filter (like a speaker will do, because it cannot move that fast).

But directly sampling the audio signal is overly complex, only for making a VU meter visualization. These visualizations are way slower, than the actual audio signal. So you need to take the mean value over a specific time interval. Best you can reach this by building a low pass filter using a resistor and a capacitor. You can sample the output of that filter, which will only have lower frequencies. The sampling frequency then should be double of the cut of frequency of the filter at minimum.

Of course it is also possible to do all this in software, but for this you would have to change the library and I guess you don't want to do this.