In 1999, music industry revenue in the United States peaked at $14.6 billion (all numbers are from the RIAA). It is important to be precise, though, about what was being sold:

$12.8 billion was from the sale of CDs
$1.1 billion was from the sale of cassettes
$378 million was from the sale of music videos on physical media
$222.4 million was from the sale of CD singles