Harvey,

Lots of music both old and new has been written using this type of
"magic square" musical numerology. Around the year 925 the musical
world had settled into a Staff of 4 lines and this was used for about
another 400 years before the the 5th line was introduced.

This type of musical thinking culminated eventually evolved into the
12 tone system of Schoenberg, Berg, Weber, and the recently deceased
Karlheinz Stockhausen. Numerology sequences were sometimes used by the
eclectic John Cage, (not to mention star charts).

J.S. Bach was famous for sneaking in his last name (h is b natural,
and b is b-flat in German) into his fugal compositions.

So could the Knights Templar have hidden messages in the music?
Surely! Just gotta pick the right tunes, I guess!

James