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.

The SAMPLE_DURATION is now passed to the `level` element of the
pipeline through the `interval` parameter.

Fixes #2532
3 jobs for master in 79 minutes (queued for 9 seconds)