Top-selling soundtrack of 1998:

Titanic
The Titanic soundtrack, featuring Celine Dion, was the top-selling soundtrack of 1998, with 61,456,000 copies sold.
Source: The Wall Street Journal, 8/11/2006

 
   
  Top-selling soundtrack of 2005:

Get Rich or Die Tryin'
The Get Rich or Die Tryin' soundtrack was the top-selling soundtrack of 2005, with 22,849,000 copies sold.
Source: The Wall Street Journal, 8/11/2006

 
   
  Top-selling movie soundtrack of 2004:

Shrek 2
With 27,367,000 copies sold, the Shrek 2 soundtrack was the top-selling soundtrack of 2004, out-selling even the soundtrack for Ray.
Source: The Wall Street Journal, 8/11/2006

 
   
  Top-selling soundtrack of 2001:

O Brother, Where Art Thou?
The O Brother, Where Art Thou? soundtrack, comprised entirely of bluegrass, was the top-selling soundtrack of 2001, with 40,529,000 copies sold.
Source: The Wall Street Journal, 8/11/2006