Since no form of radiation can ever get out of a black hole, how can astronomers detect where black holes are

Basically gas swirls around the black holes so fast that they heat up and emit rays that can be detected on Earth.

The speed of  sound:   sound travels  at different speeds depending on what it istraveling through. of the three mediums (gas, liquid, and solid)  sound waves travel the slowest through  gases, faster  through  liquids, and fastest  through  solids.