mpg321 is a clone of the popular mpg123 command-line mp3 player. It should
function as a drop-in replacement for mpg123 in many cases. While some of
the functionality of mpg123 is not yet implemented, mpg321 should function 
properly in most cases for most people, such as for frontends such as
gqmpeg. 

mpg321 is based on the mad MPEG audio decoding library. It therefore is
highly accurate, and also uses only fixed-point calculation, making it
more efficient on machines without a floating-point unit.

While mpg321 is not as fast as the non-free mpg123 on systems which have a 
floating point unit, it comes under the GNU General Public License, which
allows greater freedom to its users. For most people who want mpg123,
mpg321 is a better alternative.

WWW: http://mpg321.sourceforge.net/