You haven't indicated what you did for the low pass filter, but what I've found with these devices when I used them 30+ years ago is that you really can't see the higher LEDs turning on with a straight signal going into them, since they are on for such a short time, they're very dim. For a proper VU meter, you need a peak hold circuit - mechanical VU meters had a fairly quick rise under the moving coil's force but were well damped so returned fairly slowly under the hairspring.
The LM3915 datasheet shows a peak detector based around an op-amp, the size of that output cap will determine the response.
The gain of the op-amp circuit can be adjusted to get a reasonable scaling, but you can also reduce the value of the reference voltage on Rhi (pin 6) to increase the sensitivity at the expense of accuracy. The datasheet shows examples of that too.