Why is there a flashing red banner when I switch out of the application?

iOS displays a red banner to indicate to the user that an application is accessing the mic. This prevents applications from recording you without your permission. Hopefully a future update to iOS will present this information in a less intrusive way.

How come the auto-trigger does not work when I’m on the phone?

The iOS audio system is priority based.  The phone application has higher priority than other applications and does not permit other audio application to access the audio system when it is active.  Filtrate will resume when the phone call ends.

How come I keep increasing the normalization level but the output does not get louder?

Filtrate limits the maximum amplification factor to avoid distortion of the sound (usually from feedback).  Really low sounds will not be amplified more than maximum allowed factor.