a little rock music

What happened was, Sarah Morewood hid a music box at Balance Rock, astonishing her fellow picnickers--one of whom was Herman Melville. Sarah Morewood, who loved to devise magical entertainments for her friends, is the "cunning priestess" and "wood nymph" of Godfrey Greylock's account in Taghconic; the Romance and Beauty of the Hills, quoted below from the 1879 edition because I like the quote from Comus which the 1852 first edition lacks:

I have since heard the story of the merry hour when " Memnon" was inscribed by a hand which has written many a witty and clever volume. Indeed, indeed there must have been a deal of witchery in the cunning priestess who made that stern old rock breathe such mysterious and enchanting music.

"Can any mortal creature of earth's mould
Breathe such divine enchanting ravishment ?" [Milton, Comus lines 244-5]

I should think not. Was it a wood-nymph then with her music box!  Was there ever anything in that broken champagne bottle at the foot of the sphynx ?  And do wood-nymphs drink champagne ?  This grove is very questionable and full of marvels.

When we had clambered with a world of pains on to the top of the rock, we, too, had music — merry and sad — "music at the twilight hour."  Then, as the evening shades deepened in the wood came low spoken words of memory and of longing for those far away.

In Reading Melville's Pierre (LSU Press, 2007) Brian Higgins and Hershel Parker date the episode to the fall of 1851, probably September 26, 1851.

Well OK then.  See what happens to the Captain of U. S. Dragoons, reclining on a rock (with a lake in the background!), in the next-to-last installment of "Scenes Beyond the Western Border," Southern Literary Messenger 17 (May 1853), 313:

July 25th.—Last night I was moody and sleepless, and so witnessed several sublime and beautiful changes of weather and sky; such as, indeed, many scarcely notice, and few in houses observe,—as in cities and towns they rarely can; and they were accompanied by an incident, as startling as delightful, in my ignorance that there existed in the camp any means to produce it.

The labours of the day, the duties of the evening, all over, sleep had followed, as the labourer's luxury:  lights had gone out; the little fires sunk and paled; sounds gradually died away; the tents gleamed strangely in the moonlit solitude. I would have taken refuge from my thoughts in sleep; but sleep often flies us when most invoked.

At last I wandered forth alone, and ascended the mount.

The moon, not yet full, was high in heaven; the deep shadow of the pines slept on the grassy mountain top; the little lake below brightly mirrored the glittering sky; now and then came deep breaths of air,—like sighs from the gentle heart of Night. Long I reclined motionless upon a rock: I was alone—there was no sight or sound of past or present life—but had no sense of loneliness; for the soul felt not a motive, and the heart seemed dead. Vain were the silent appeals of beauty; —vain even the solemn dirge-notes of the pine forests.

At last I gave a deep, involuntary sigh. Just then, O strange, upon the mountain top! —as if in answer, came, gently stealing on the air, a strain of soft music. This Heaven-bestowed key to all hearts, and to all moods, aroused in me some of that life which silence and solitude so profound had absorbed. It was like an exquisite dream closely following the last weary and oblivious sense.

But soon, the music changed; and, stranger then and there, to a sweet waltz!

Then swiftly awoke Memory, to make it an echo of the Past; and vigilant, prisoned  Hope stole forth trembling, like the moonbeam on the little lake.

Something made the music.  The Captain hints as much in the original magazine version, when he explains his surprise as mainly resulting from his lack of information:

"in my ignorance that there existed in the camp any means to produce it."

Meaning his ignorance that somebody brought a music box?  This important confession of ignorance does not appear in the corresponding passage in the later book version, Scenes and Adventures in the Army (Philadelphia, 1857).  Wow!  Comparing versions I just realized the significance of another alteration to the text as originally printed in the May 1853 Southern Literary Messenger.  In the original magazine version, the music is "a sweet waltz."  Or rather, the music starts one way then changes "to a sweet waltz."

But why transform the waltz into the vaguer "joyous air" for the book version?  Same reason you cut the reference to some real, tangible "means to produce" the music.  Taken together, both references, to practical "means" of making music, and to the specific type of music produced, "a sweet waltz," evoke the likeliest mechanism for producing waltzes in the wilderness...