Is it permissible to use earplugs that reduce the intensity of sounds during acts of worship, such as prayer and reciting the Quran, in order to achieve humility (khushu') and contemplation, or is this considered an innovation (bid'ah)?

If earplugs are worn to ward off disturbance during prayer or recitation of the Holy Quran, and not as an act of worship or a belief in their legal specificity, then this is not an innovation (bid'ah). This can be analogized to closing the eyes during prayer if it helps in achieving devotion (khushu'), as some scholars have stated that it is not disliked, but rather may be recommended in this case to repel what disturbs the heart and prevents devotion.