Add listener for background noise to dynamically update levels

Speech is hardcoded for a level below -2000 and above 2000 but not more than 7000. Background noise is determined to be above 7000. This leaves little room to detect speech, and the user has to speak clearly and loudly over the background noise.

These numbers should be variables depending on a small background noise sample. A few seconds should give an average upon startup, and perhaps when silence is detected it could be updated -- at some interval.

This would allow louder backgrounds to be omitted as well as quieter speech to be understood.