Have you tried the computeSpectrum() method of the Sound class? Since flash V9.x it can compute the spectrum on RTMP. The link to the API has a full example that makes a waveform. It computes a value from -1.0 to 1.0 for 512 values, 256 for the left and 256 for the right channels.
You can bundle them in blocks and use the data to compute your audio levels. Just bundle a bunch of values and make one bar that displays the average of the values to simplify it. e.g. 4 bars uses the average of the values from 0-64, 65-128, 129-192, 193-256 for the left channel.. You get the idea.