JCraft hasn't responded to my email yet about the error that happened when I switched 1 of my sound files from mono to stereo.
About the 1-ear bug, I talked to someone from OpenAL-Soft. We figured out the issue was that in version 1.14 of OpenAL-Soft, released March 28, 2012, OpenAL-Soft switched from using a dsound backend to an mmdevapi backend. The issue was the difference in the way the 2 backends detect the audio settings.
Dsound uses the driver, while mmdevapi uses the Windows settings. My headset is detected by Windows as a 7.1 headset, even if the headset driver is set as stereo. So in the older version of OpenAL-Soft, my headset was detected as a stereo headset and mono sounds upscaled to both ears. In the newer version of OpenAL-Soft, my headset is detected as a 7.1 device, so sounds only play in the right ear. If I set my headset driver to 7.1, the mono sounds play in both ears in the latest version of OpenAL-Soft, and thus the latest version of LWJGL. If I set Windows to think my headset is stereo and I set my headset driver to stereo, mono sounds play in both my ears.
In the end, it's not even OpenAL-Soft, it's dsound vs mmdevapi.