A simple Linux sound meter based on ALSA API. Here's the demo on youtube. The noisy sound is generated by a guitar tuner, and as it gets louder, the dB and peak value displayed on the screen gets larger.
Just run make, it should work (I hope).
I wrote this program few years ago and I haven't wrote C and ALSA stuff for a while, so I might not able to answer your question, but I'll do my best if you got any! :)
Please visit ALSA project - the C library reference for more information. This code is based on
alsa-lib/test/pcm_min.c.