From the README:

  Xmmix is a sound mixer utility for the X Window System using the
  OSF/Motif interface.  It requires the Unix Sound System (formerly
  "VoxWare") sound driver by Hannu Savolainen (hannu@voxware.pp.fi).

  For those of you with access to the WWW (via Netscape, NCSA Mosaic
  or other WWW browser), The URL for the xmmix home page is as
  follows:

      http://sunsite.unc.edu/~cddb/xmmix/

  This home page provides up-to-date information about the
  current release version, online README file, FTP service to get
  the software, an interactive form to send comments and
  suggestions, and late breaking announcement, if any.

I don't have hardware to test all the features, but it looks ok.
eric.
erich@FreeBSD.org