I was looking at Essentia and it said it is compatible with Python and JavaScript but I am not sure if this is compatible with Processing or not? This library deals with audio. Here is the link to its Github repo https://github.com/MTG/essentia
If it is compatible with Processing, from the audio file, am I able to get its tempo/ beats per minute, key signature or pitch and the loudness of the song?
It says that it is a C++ library.
I feel like you should try researching, googling and reading a bit before posting another question about sound libraries 
look at minims functions maybe you’ll find what you are looking for:
http://code.compartmental.net/minim/
http://code.compartmental.net/minim/beatdetect_class_beatdetect.html
http://code.compartmental.net/minim/audiooutput_method_settempo.html
http://code.compartmental.net/minim/audiooutput_method_gettempo.html
http://code.compartmental.net/minim/audiooutput_method_getvolume.html
Edit: looks like getTempo() doesnt really work. I dont know if I tried it wrong tho.
That is literally
what I have been doing and there doesn’t seem to be anything easy to use, either that or support Processing.
Yes, using getTempo() does not work for me as well. I have been really trying to find out what to use as an alternative but its very hard to find.
By the way I am not interested in setting the tempo, I want to only get the tempo. But anyways thanks for the help.
I said this because it says it is a c++ library on the first sentence of the github repo. And if you go into their site, there is a documentation available and it says getting tempo is one of its features. 
But they do support Python as well that is why I was a bit confused, nothing more 
If they support phyton, you could try it with processing.py. It’ll work maybe? I don’t know much about phyton libraries.
Thanks anyways mate 