Renaissance music would be a complete cop-out. Remember, the renaissance is considerably closer in time to today than to Arthur's era. Even Gregorian chant is way too recent for Arthur! I'd agree that we know nothing about what this music might sound like, but harps, pipes, whistles, horns and drums would be your best bet. Noseflutes, too, and, as a close perusal of Monty Python and the Holy Grail would tell you, butt-trumpets!