Bitstamp was audited by Mike Hearn, a Bitcoin dev back in May 2014. He said that everything seemed OK and all the funds were fully backed in their cold storage wallets. This was just 8 months ago and I'd be surprised if the situation has changed since then.
From what i can read this only pertained to the BTC balance, not the USD balance.
On the other hand i don't think Pantera would have invested $10 million without a proper audit.