I am looking for a tool that will tell me, in less than half a second, if the microphone is picking up any sound above a certain threshold. (I plan to then mute the Master channel with another command line tool, like amixer.)
This solution will avoid writing repeatedly to disk, and even though it in worst case takes a second instead of the desired less than half a second, I found it to be fast enough after trying it. So, here are the two scripts I use:
Hardly elegant, but it works.
Note: If you want a more gradual thing, add something like this:
for muting and