When starting Spiral Knights, only music plays.

I've tried to changing from java 1.6 to java 1.8 (reinstalling afterwards), I've disabled the steam overlay, and I've verified the integrity of the game files several times.

An odd behavior: when I verify integrity via steam, it says 15 files needs to be replaced. Then, when I start the game, it updates. Checking the integrity after that shows that 15 files need to be replaced.

Any thoughts?