Yes, I guess most of it is lost, but I do believe musicologists are still discovering "new" music in old and forgotten manuscripts scattered in the libraries of Europe.
Here is the third mode melody of Thomas Tallis for Archbishop Parker's psalter. In my opinion the most beautiful melody ever written (at least the most beautiful I've ever heard). Vaughan Williams' Fantasia on a theme by Thomas Tallis (my most favourite piece) is based on this this psalm chant tune.
Third Mode Melody
Alexandros