The Arnia Project is the first commercial hive monitoring project based on audio data. They have some interesting scientific papers about sound recording and analyzing published.
The excellent videos give a nice insight to the developed hardware, especially the scale design:
http://www.arnia.co.uk/using-the-hardware/
An older article for a first overview:
http://www.beeculture.com/arnia-remote-hive-monitoring-every-beekeeper/
I Like the insights they gather from the entrance audio analysis: After the Swarm: Will the bees survive and prosper? | Arnia
Depending on the audio level peaks they can show when the mating flights have taken place. I have also seen interesting findings with an entrance microphone on other projects.
A nice presentation from Huw Evans with a brief overview about sound monitoring Arnia Remote Hive Monitoring - Huw Evans.pdf via apimondia.com