Yep - doable.
The trick would be to calibrate the sound level. For that you may need a sound level meter of some sort which isn't on the Pi.
The idea would be to do the following :
1] Use a sound card with a microphone either USB or GPIO.
2] Either write some software to implement a sound level meter or search for "ALSA sound level meter" and use some pre-existing software.
3] Calibrate the system using the sound level meter. You would have to see what the sound level meter software produces in dB SPL for a particular sound and find the difference between the sound level meter and the Pi's software sound level meter.
4] Work out how to get the output from the software sound level meter add the difference you found in step '3]' to get the actual dB SPL and activate one of three GPIO pins which drive some LEDs of some sort ... red, yellow and green. You will need to work out what thresholds you need to switch between colours ... 60 dB SPL is strong speech, 120 dB SPL is a jet airplane up close, here are more descriptions
5] Work out how to set the GPIOs high and low in order to implement step '4]'.
Sounds like an involved but doable project !