In the scene pictured above, Kirk and Spock are obliged to take public transit and find themselves sitting across from this charming individual who is loudly playing what might be termed music on his ghetto blaster. He responds to Kirk's pained request to turn down the volume by obnoxiously cranking it instead, and Spock solves the problem with a well placed Vulcan neck pinch.
Incidentally, the punk is played by Kirk Thatcher, who was an associate producer on Star Trek IV. He also wrote the lyrics to the song I Hate You being played on the blaster and provided the vocals for it, backed up by some members of the sound department.