Rework audio processing to avoid seeking past end of file

In some cases when there is a slight mismatch between audio file
duration and event timestamp difference, and we have a record
status or chapter break event in a certain location, it could
trigger a seek past the end of an audio file. Detect this
condition and just render silence instead.

Also adjust the thresholds for the audio length scaling - they
were being triggered on short recordings that should be correct.