“What’s interesting is that if this was true: No music or singing or playing allowed at all, how would anyone know what music was?”
There are a lot of goyim in the world. Also Jews who are tinok shenishba or off the derech. Even if we weren’t allowed to listen to music, you couldn’t help hearing it on the busses and in the stores and coming from your goyish neighbors’ houses.
Also, I think it was always allowed at chasunas, etc.
And, I think you can miss something even if you don’t know what you are missing. Like we are missing something because we don’t have the Bais HaMikdash even if we don’t realize what it is that we are missing. People turn to drugs and alcohol and other such things because they are missing something in their lives (love, Torah, connection to others and to Hashem) and they never had it so they don’t know what it is that they are missing, but they know they are missing something.