Only a single fluid exists at the liquid-vapor critical point; it exhibits properties of both phases.
When a liquid has been heated to a temperature above its boiling but still doesn’t boil, it’s superheated.
If a liquid is cool rapidly enough it can miss the freezing point and end up as a supercooled liquid.
While all glasses are amorphous solids, not all amorphous solids are glasses; glasses exhibit an actual glass transition. Even though we say solid, a glass is actually a very viscous liquid that moves very slowly over time.
Unlike the crystal an amorphous solid or glass is a disorder arrangement of the atoms of the condensed phase.
At some temperature a supercooled liquid transitions to a more solid or condensed form; this is the glass transition temperature.
The glass in a window (for example) is really a very viscous liquid that moves very slowly over time.
The speed of sound depends on the phase of the material (solid, liquid, or gas). In general, sound travels fastest in solids, then liquids, then gases.