Silence-detection does not read configuration value for ffmpeg binary path

Steps to reproduce:
1. install ffmpeg to a different system binary location (e.g /opt/ffmpeg)
2. set org.opencastproject.composer.ffmpeg.path=/opt/ffmpeg/ffmpeg in configuration.properties
3. run silence-detection

Actual Results:
silence-detection fail, because ffmpeg binary istn't found on your system

Expected Results:
silence-detection should use ffmpeg binary from config.properties

Workaround (if any):
add ffmpeg-path to $PATH