Slit gongs are made of wood, which doesn't last for thousands of years in playable condition. (The oldest wooden Polynesian object I know of is the statue of Uenuku, probably taken from central Polynesia to New Zealand around 1350 and maybe a few hundred years older than that). There are slt gongs that predate the Europeans but probably not by very long.

Bronze casting postdates the Macedonian ocarina so no bronze bell can be older.

Stones are a good call. Bob Pegg uses one in his routine, a pebble with a wormhole through it eroded by a smaller and very hard pebble under wave action. You can blow it like a shepherd's whistle. There must be stones like that that were formed before life evolved on dry land to blow them. But we can't tell which they are. And lithophones go back a long way:

http://www.lithophones.com/index.php?id=2

I once heard a broadcast of a performance by a Chinese lutenist using two moon lutes both more than a thousand years old. He had toured the world with them, but wouldn't risk taking them on a plane because of the risk of sudden pressure and temperature changes. They sounded wonderful and it was easy to understand why somebody looked after them for all that time.