Will seperate this into two parts:
Korea/China actually have the "soundproof booths" for players, but not because of what you think. Apparently in the early stages of those leagues, some teams would actively try to cheat via special "codewords" (like if someone in the crowd yelled "banana", it could mean that the enemy team is doing baron). The booths were brought in as a countermeasure, and has since served the region quite well.
As for how well the players can hear the crowd, it really depends. I have heard from a few retired players that apparently with the headsets/in game sounds, hearing the audience during LCS/"regional" games can be relatively hard. During pauses or breaks though, the audience can apparently be heard so well that the players can hear what people are saying in the 4-5th row.