measuring the level of sound

Hello,

I have the following problem.

I need to develop the software to measure the level of audio signal to make a 
decision when the sound level falls below some threshold (e.g. signal from mice 
disappear or people stop talking in room and leave it).

Can anybody point me on how to do this? Any help (link, theory, code, etc.) 
would be appreciated.

Thanks in advance.

----
Mykhaylo

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