The circuit is too simple. It is missing the peak detector circuit shown on the datasheet that makes the LEDs look bright and good.
Your vision cannot see a brief peak brightly, just dimly or nothing. The peak detector circuit holds it long enough to be seen.
I think you had the volume turned up too high so the output clipped severely producing square-waves that were either maximum high voltage or nothing. A VU meter is supposed to be linear.
Your LED display simply turns on all LEDs then turns them off. Sometimes the LEDs turn on and off so quickly that you don't see them or they are very dim.
A VU meter shows the different levels of music.
The LEDs of an LM3915 light at 3dB differences.
Your LED display simply turns on all LEDs then turns them off. Sometimes the LEDs turn on and off so quickly that you don't see them or they are very dim.