Do not add coffee grounds or eggshells as a maintenance treatment. Whether a small amount is permitted as ordinary waste is a separate question answered by the exact model's food list.
What the observation tells you
A disposer is not a kitchen knife that needs sharpening with gritty ingredients. Claims that abrasive scraps clean every appliance overlook the outlet pipework and the possibility of leaving more particles there. A quieter sound after adding material is not proof of a repaired grinding mechanism.
A useful next step
Use the manufacturer's cleaning method. Keep these materials out when the instructions are unclear, and have declining performance investigated instead of trying abrasive food remedies.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.