NEXT weekend, Manorbier's classical music historian Paul Griffiths reaches midway point for the year in his series of public appearances with a pre- concert talk for the Proms on Monday, July 28, at the Royal College of Music.  The talk will be recorded and posted at 'Proms Plus' on the BBC website, as will the programme notes Paul wrote for 15 of this summer's Proms. Other speaking engagements in the past six months have included presentations for the BBC Philharmonic Orchestra in Manchester, for Lincoln Center (New York), for the City of Birmingham Symphony Orchestra, for the BBC programme 'Hear and Now' and for the Aldeburgh Festival. In the autumn, Paul will go again to the United States, principally to give the Messenger Lectures at Cornell University, but also to accept invitations in California, where he will teach a class at the San Francisco Conservatory of Music and participate in centenary celebrations at Stanford University for the French composer Olivier Messiaen.  Messiaen's compositions for church organ are among the greatest works of 20th century sacred music and have been performed at St. Mary's in Tenby. Later, he will travel to Stavanger in Western Norway, again to take part in Messiaen festivities.    Also this autumn, Paul will add five books to his list of publications: translations into Spanish and German of his Concise History of Western Music; a translation into French of The Sea on Fire, his biography of Jean Barraqué; a reprinting of his 1985 book Olivier Messiaen and the Music of Time, in honour of Messiaen's centenary; and a new novel from Reality Street Editions.  In addition, the Boston Symphony Orchestra has released a DVD of the opera 'What Next?' with words by Paul and music by the US composer Elliott Carter.  This year, the works of Carter - born the day after Messiaen in December 1908 - are  being played in centenary concerts all over the world; the music of both composers was featured on the first night of the Proms and will be heard several more times during the festival. The difference is that Carter himself sometimes appears in the audience, fit and still composing music at age 99-and-a-half.