previewers: More precise audio waveforms

Even though SAMPLE_DURATION was set to 1/100th of a second, we were only collecting peak level data every 1/10th second and then average the rest of the unknown values. In this commit we matched the SAMPLE_DURATION and interval of data collection. This produces more detailed waveform and removes the need of averaging unknowns.

Fixes #2532 (closed)

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