The idea here is that the speed of sound in more dense object is more. for $e.g.$ $v_{Aluminium} = 6420 m/s$, $v_{water} = 1498 m/s$ and $v_{air}= 346 m/s$

As the density of medium goes on increasing the speed of sound in that medium increases. Black holes are very dense objects, then what can we say about the speed of sound in black hole