Is it permissible to listen to audio recordings of ruqyah inside a bath drawn with water infused with 'sidr' (lote tree leaves), while in the bathroom, using headphones?

There is no harm in bathing with water containing Sidr.

As for listening to the Quran in the bathroom, it is permissible if the sound source is outside it, but forbidden if it is inside, out of reverence for the Quran and the remembrance of Allah. It was reported that the Prophet, peace and blessings be upon him, used to remove his ring, which had the name of Allah inscribed upon it, before entering the lavatory. Scholars have also unanimously agreed upon the desirability of removing anything that contains the remembrance of Allah before entering the lavatory.

Sheikh Ibn Uthaymeen issued a fatwa permitting the placement of a recorder outside the bathroom to listen to the Quran, and mentioned that the grandfather of Sheikh Al-Islam Ibn Taymiyyah used to do this to save time, though it is preferable to avoid it so that one does not prolong their stay in the bathroom.