On English translations of this hymn, as well as other information, Wikipedia is pleasantly accurate.

Although often attributed to Bernard of Clairvaux, the Latin originaln Salve mundi salutare, is now attributed to the Medieval poet Arnulf of Louvain (died 1250).

Paul Gerhardt translated the entire original poem in to German.

The first English translation of "O Haupt voll Blut und Wunden" in 1752 was by by John Gambold (1711–1771), and begins with the words "O Head so full of bruises."

The American James W. Alexander's translation in 1830 begins "O sacred head, now wounded."

The 1861 English translation by Sir Henry Baker, from the Gerhardt German version, begins "O sacred head surrounded by crown of piercing thorn."

The Robert Bridges 1899 translation, from the Latin original, begins "O sacred Head, sore wounded, defiled and put to scorn."

The melody, by Hans Leo Hassler, was written around 1600 for a secular love song, "Mein G'müt ist mir verwirret", which first appeared in print in 1601.

In 1656, Johann Crüger took the Hassler tune and used it, rhythmically simplified, for Gerhardt's hymn translation.

Bach used five stanzas of the hymn in his St. Matthew Passion in various arrangements (rhythmically and melodically) and harmonizations. Stanza 6 appeared in his cantata Sehet, wir gehn hinauf gen Jerusalem, BWV 159. The same tune appears in Bach's Christmas Oratorio, but with entirely different words.

We used it at the Offertory at the English masses with Organ & Cantor.

i am sore surwounded that we aren't singing this hymn

Marajoy -
Sore is also used as an adverb or even an adjective, particularly historically.
I might say of a friend that 'I missed him sorely', meaning quite a lot, or grievously.
Or, there is the King James rendering of the angelic greeting to the shepherds, who were 'sore afraid', meaning 'very' or perhaps 'hurtfully'.
If you were rather put out about something, someone might say that you were sore vexed.

The OED says:

sore, adv. Now chiefly arch. and dial.

1. Of striking, wounding, etc.: So as to cause considerable physical pain or bodily injury; violently or severely in this respect.

In typical modern use this is indeed characteristic of careless or dialectical speech, and so you will see things like "I was awful scairt, ma'am." But this is not always the case. The -ly ending was formed from the word "like" (Anglo-Saxon lic), so stoutly = stout-like, fully = full-like, etc. However, sometimes adverbs naturally formed from adjectives without the -ly. Fowler explains how:

In Anglo-Saxon the adverb was usually formed from the adjective by the addition of e; as, Georn and geornlic, earnest; georne and geornlice, earnestly: sweotol, manifest; sweotole and sweotolice, manifestly: swidh and swidhlic, great; swidhe and swidhlice, greatly.

It is exactly by dropping this adverbial termination e, especially where the connection in meaning of the adverb and of the adjective is not immediately obvious, that in English many adverbs are found agreeing in form with the adjectives from which they are derived; as,

Anglo-Saxon lang or long, adjective, and lange or longe, adverb. Whence English long, adjective and adverb.
Anglo-Saxon laet or lat, adjective, and laete or late, adverb. When English late, adjective and adverb. Compare also clean, cleanly, fast, high, hard, loud, right, sore, soft, thick, wide, ready, evil, ill.

That this mode of forming adverbs has extended itself to a few adjective of French or Latin origin is not surprising, as, English clear, adjective and adverb (compare French clair, Latin clarus and clare); just, adjective and adverb (compare French juste, Latin justus and juste.)

The apparent poetic use of adjectives for adverbs is to be explained on the principle that the poets delight in antique forms. This use is often found in poetry, where it is obsolete in prose . . . .

Mark Thompson -
Many thanks for this offering from OED and Fowler. Though I am no German scholar, I took note of the similarity in the German adverbial forms ending in -lich (-like), which equals an extant paralell usage similar to the Anglo-Saxon.
Then there is the current teen-age street language usage of putting 'like' before rather than after a verb to conjur up what would be an adverb.

As The Giffen wrote:

The melody, by Hans Leo Hassler, was written around 1600 for a secular love song, "Mein G'müt ist mir verwirret", which first appeared in print in 1601.

While Hassler wrote what we call a Waltz, they were instrumental works.

There's more at:

http://de.wikipedia.org/wiki/Mein_G’müt_ist_mir_verwirret

Use Chrome or another browser (chrome auto translates) and let google translate the web page for you, if only for the chance to see the title:

Just a note for EF choirs, there is a useful Latin version of this tune available from CPDL, Haec tibi dona pura. From what I can tell, the Latin verses are about sacrifice and redemption and should be appropriate for all seasons as an Offertory motet.

HERE is a link to the pdf on cpdl.org.