When you try to play your selection of (clearly) well-chosen music at some event, yet somebody who has a collection of vapid (most likely pop) music interrupts you and their music takes precedence for whatever reason(usually because it's more popular, particularly amongst teeny-boppers or preps). The effect is more dramatized if your music was played for a few seconds before somebody stopped it.
Person A: Hey, I'm going to plug in my mp3 player so we can listen to some Talib Kweli.
Person B: Nah, let's just use my iPod and play Miley Cyrus - Party in the USA.
Person A: Oh wow, I got audio cockblocked...