Clairaudience is not developed through "spells". I spent half a year developing it through meditation. Its basically when you quiet your mind and allow thoughts to flow. Ask a question to a spirit and listen to what your thoughts tell you.
It can be hard to detinguish the difference between your own thoughts and the clairaudient messages being sent through your mind. There are few exercises you can do develop it.
1. Listen to classical music and try to listen to one instrument group at a time.
2. Listen to the sounds all around you, even the sounds that you usually ignore.
3. Actually try to listen to what the spirit is saying like I described above.
At first the voices might be in bursts of a few words, but eventually you will be able to hear full sentences.
For me, it was a natural development of my abilities. I started off seeing spirits when I was five. I ignored it for a long time before accepting the seeing part. I would see their mouths moving, but not actually hear anything. After a TON of meditation, I finally started to hear them sometimes. It started off sounding like a room full of people talking at once, buy gradually became clearer. 8 years later, I still see a spirit several times before I hear them, and when I do hear them, it's sometimes difficult to understand.