An idea that is occasionally bandied about is that the Gaelic harp traditions of Ireland and Scotland, as old and conservative musical traditions, may fossilise the remnants of much older prehistoric traditions that may have been widespread all over Europe.
Certainly there are very ancient mythic strands in the Gaelic traditions that seem to have been abandoned by the more recent, more progressive musics that form the mainstream of the Western tradition - church chant, musical notation, the development of formal harmony, leading ultimately to Classical music and modern 'common practice'.
It is a slippery subject area not least because of the 'Celtic mist' that addles the mind of anyone foolish enough to get interested in these subjects; the layers of myth-making, some ancient, some modern fakes, are very hard to peel apart, and one is far more likely to discover ones own modern cultural biases and desires than any ancient reality.
Ann & Charlie Heymann have done some good background work1, connecting the old Gaelic harp traditions with Dumézil's threefold division of society, with old beelore, and other interesting cultural strands. An interesting direction is the idea of cult animals - perhaps most obviously the enigmatic double headed fish appearing on the forepillar of many harps.
Early Irish myths and legends sometimes mention harps in the most ambiguous terms possible. The most celebrated is the story of the Dagda and his enemies the Fomorians:
It is hard to know what to make of a tale like this but that does not stop people trying!3. We might briefly note the ahistoricality of the story, a 16th century text, probably compiled in the 12th century from 9th century elements, telling of ancient pre-Christian battles between the gods. Uaithne is named either as the Dagda's harp, or as his harper. The Dagda invokes the harp, or the music, with such power that it kills men as it leaps from the wall, and he plays the 'Three Strains' of music to physically enchant the enemy as they make their escape. The incantation is probably corrupt; it may refer to the component parts of the harp, or to elements of musical structure.
If one is willing to speculate wildly, one can start to imagine the nature of the supposed prehistoric underlay of European culture. Based on high status, magically-enhanced stringed instruments played by priests or kings, with close connection to the public declamation of poetry for pagan religious ritual or political and historical effect, we can imagine the instrument being considered somhow a sacred structure, with every component part named and loaded with mysterious significance. We may expect the tuning and musical structures to be connected to the ancient Greek systems, or we may even suppoose that the Ancient Greek musical theory was a pale shadow of our much more ancient mythical worldview.
It's important to keep a sense of proportion however. My own sense is that the medieval scholars and chroniclers loved this kind of wild theorising, and that is as good a spirit as any to enter in to it. However we must also remember that there is a real, solid tradition, and there is no point flying off on fanciful theories if we ignore the historical evidence and the attested historical practice of the old tradition.