How can a deaf player detect where monsters and caves are?

Most of the monster sounds are low pitch. There are chairs that integrate speakers to let players feel low frequency sounds. You should investigate those.

The Captioning API is a modloader mod that captions the sounds of the world. Some of them are deliberately ambiguous, I only learnt through hearing the noises at the same time.

You could turn off all music and turn the sound volume up. Lay the speakers on their back, put a thin plate on them and put sand on them. Sound vibrations will cause Chladni Patterns (sorry, the German Wikipedia Article is more informative here...) to occur, similar to the ones you see in this YouTube video:

With some practice, your niece might learn to differ between the patterns of e.g. Zombie grumbling and water flowing.