It's filled mostly with papers, scientific notes. There's also $500 in currency. Yes, and a couple of postage stamps.
8 seconds sound clip from Superman (1948) - Man Of Steel (Chapter 4).
You can hear this line at 00:05:45.095 in the Blu-ray version of the episode.
Quote context
[...]
- You don't believe me. Give me a test.
- If you are really Superman, you have x-ray vision. Tell me what's inside that closet.
- My x-ray vision has been destroyed. Must be the effect of that meteorite.
- Try the safe. It's only steel. Tell me what's inside.
- And now, if you want further proof...
- You're the only one who knows.
- Your secret is safe with me.
- Now that I'm convinced, what do you want me to do?
- Washington has selected me to guard the relativity-reducer ray.
[...]