Fix some shortcomings of the audio level estimation

Use crude approximation of the equal loudness curves (a single 2nd
order bandpass at 1 kHz). Because we must be stateless, the filter
is always reset first, which somewhat damages the behavior.

Limit adjustment range to 30 dB to avoid problems when sound goes
very quiet.

Make ramp up fast and ramp down slow. This gives reasonable response
to transients between periods of silence.