viftool splits tomtom .chk files into .ogg files and vice-versa. it is a commandline tool so you'll need to open a command prompt (<Win>+R , "cmd" , <CR>) to use it!
2011-08-15 : current windows exe version. supports tomtom 920T.
2006-01-11 : fixed windows exe version (fixed cr/lf bug in ogg extraction)
2004-09-14 : first windows exe version
This is the sourcecode for the latest version. you'll need a python interpreter to run this.
2008-08-23 : sourcecode (python / linux, windows, mac os; patched for 920T)