AMONG the small group of new Christmas classical releases this year are several noteworthy - and sometimes offbeat - albums suitable for serious music lovers and casual listeners alike.One release getting prominent promotion - and justly so - is a collection of traditional songs and hymns sung by Thomas Hampson, a young American baritone receiving growing acclaim in the United States and abroad. "Christmas With Thomas Hampson" (Teldec) features the Saint Paul Chamber Orchestra and conductor Hugh Wolff, an up-and-coming star on the US conducting scene. Perennial favorites such as "O Holy Night" and "The Little Drummer Boy" are treated with refreshing sensitivity rather than schmaltz, while ancient German tunes "In dulci jubilo" and "O du frohliche" stand out for their beautiful arrangements and Hampson's clear conveyance of text. Though some secular selections like Berlin's "White Christmas" cheapen the mix, this holiday compact disc is worth adding to any collection. Flutist James Galway teams up with the Munich Radio Orchestra on "In Dulci Jubilo: Christmas With James Galway" (RCA Victor Red Seal). Don't expect stunning showmanship from the master on this one; the selections are straightforward presentations of Christmas carols and other favorites. A few less-worn pieces add interest, though, such as Mendelssohn's charming "Six Christmas Pieces" and Brahms's "Prelude on 'Es ist ein Ros' entsprungen. One new recording of Handel's "Messiah" (Harmonia Mundi) takes the "historically informed" approach to a new plateau. The Philharmonia Baroque Orchestra and the U. C. Berkeley Chamber Chorus have painstakingly recorded all of Handel's variant versions of specific pieces within the work. Listeners with programmable CD players can substitute among the 18 alternatives and re-create, with the help of charts in the CD booklet, up to nine different concert versions performed during Handel's day. The three-disc set is capably conducted by Nicholas McGegan, who adds some effective interpretive touches. A real symphonic treat this year is a new release of Hely-Hutchinson's "Carol Symphony" (EMI Classics) recorded in 1966 by Barry Rose and the Pro Arte Orchestra and compiled anew with splendid Christmas works by Vaughan Williams and others. Hely-Hutchinson, who lived in the early part of this century, weaves familiar carols into a lively and entertaining work that young people will enjoy. Rose also conducts the excellent Choir of Guildford Cathedral in four serenely beautiful carols by Vaughan Williams. Fans of the crystalline tones of boy singers will enjoy "Christmas in Vienna," by the Vienna Boys' Choir (Philips). The 10- to 12-year-olds perform accompanied and unaccompanied renditions of old German hymns, carols, and shepherd's songs. "Stille Nacht, heilige Nacht" ("Silent Night") is one of a few pieces that showcase individual soloists of remarkable talent. Many of the songs tend to be tediously uniform in character, but a few numbers such as the harmonically rich "Maria durch ein Dornwald ging" and the humorous "Heissa, Buama" perk up the collection. If you're bored with standard Christmas melodies, try some Baroque and Renaissance music, which is bound to impress the ears of holiday dinner guests. Il Giardino Armonico, an early music ensemble based in Milan, Italy, has brought together "Christmas Concertos" (Teldec) of 17th- and 18th-century Italian composers such as Corelli, Vivaldi, Torelli, and Antonacci. The works are varied in nature and performed with an expressivity all too rare in modern early-music interpretations. Newcomers to "authentic" performance will have to adjust to the drier tones of the old string instruments and the players' avoidance of fat vibrato, but these are qualities that add to this music's distinctiveness. A bit more offbeat is "In Natali Domini" (Vivarte/Sony Classical), a generous collection of Christmas music from the late Middle Ages to the early Renaissance. The title translated means "At the birth of the Lord" and is one of several 15th-century cantios, or songs, on the recording. Singing and performing on fiddles, viols, recorders, bagpipes, sackbuts, organ, and dulcimers, the German ensemble Niederaltaicher Scholaren presents purely liturgical chants and hymns, as well as more freely composed "tropes," or elaborations, of religious works. A useful complement to the disc is a thick booklet containing scholarly essays, descriptions of each piece, and English translations of the texts, which are sung in German and Latin.