Poor Curiosity, all alone out there in the vast Martian wastelands with no one to hang out with on its first birthday. So what's an intrepid rover to do? Sing "Happy Birthday" to itself, that's what.
In commemoration of Curiosity's first year on Mars, NASA transmitted instructions to the rover's SAM (Sample Analysis Instrument) module. This device can create audible tones by resonating at varying frequencies. Normally, the resonance helps SAM to filter finely-grounded samples of the Martian surface for analysis.