Fiber optic cable isn't affected by corrosion since it's a hollow tube for light to travel through,
No metal no corrosion. Dust isn't an issue since once you plug in the cable there's no way for dust to get in. If there were dust inside it would stop the sound but not degrade it.
Unless the speakers in the soundbar need replacement or repair there isn't any way that the sound would have degraded over time. Maybe it's time for a better soundbar.