since JKR herself said in Quidditch Through The Ages that no wizard has ever devised a way of flying without aid, and since she basically said that wizards cant fly with out support, i think its safe to say that
voldemort wasnt really flying. but i was slightly outraged when i read that he was in Deathly Hallows. but then i thought about it, and decided he DID have support.
I think
Voldemort enchanted his robes or cloak with the same spells they put on broomsticks. i can;t see
Voldemort riding a broom, he would want to stick out, be impressive. the only way to accomplish this feat would be to fly without any visible means of support, hence enchanting his clothes. and yeah, it is against the ministry rules, but since when has
Voldemort ever followed the rules?
oh, we we do see him fly more than once. he flies when we first see him, and he also flies up to the top of the tower where Grindelwald is being held in Nurmengrad.
"'-Time to fly....'...'--He rose into the night, flying straight up to the windows at the very top of the tower---'" (Deathly Hallows, US ed., ch. 23, pgs. 453-454)