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

I see all the time that the recommended ways to mine is to dig deep and listen out for the sounds of monsters in caves.

My niece is deaf, and whilst she loves the building aspect of the game, tasks like "finding caves" or "knowing a monster is behind you" are obviously more difficult.

Are there any settings or mods that can be used to give a visual indication that the player is in the vicinity of mobs? Especially something showing the direction the sound is coming from.

In Minecraft release 1.9+ you can enable subtitles in the corner of the screen, which show what sounds played in the last ~5 seconds! Each sound subtitle fades away with time, darkening and vanishing in a few seconds. Plus, small arrows indicate which side of the player the sound came from.

Or use commands (more reliable, but harder):

To be notified when a specific mob is close, put the command below in a normal command block on a slow Redstone clock:

/execute @e[type=MobName] ~ ~ ~ execute @a[r=R] ~ ~ ~ say @p Warning: MobName

You can change the MobName to detect mobs with that name and adjust the radius (r) to a value that you want. But watch out - the command syntax is case-sensitive! Don't put spaces inside the brackets and don't forget to use capital letters in mob names!

Example: To get warned when a Zombie is less then 5 blocks away from you, use this:

/execute @e[type=Zombie] ~ ~ ~ execute @a[r=5] ~ ~ ~ say @p WARNING: Zombie

Example #2: To be notified when a cave is somewhere around and when it's close, use this bat detector (I used a higher value of r):

/execute @e[type=Bat] ~ ~ ~ execute @a[r=16] ~ ~ ~ say @p Cave 20 blocks away!

/execute @e[type=Bat] ~ ~ ~ execute @a[r=8] ~ ~ ~ say @p Cave 10 blocks away!

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.