Is it permissible for the fiancée to see pictures or video clips of her fiancé, or to hear his voice? And is it permissible for him to hear her voice or see her picture, knowing that the engagement does not involve a valid religious contract?

It is permissible for a woman to see a man and hear his voice, as long as it is not with lust. If it causes unusual feelings, caution must be exercised, and one should turn away from it to avoid fitna (temptation/strife). It is permissible for a man to hear a woman's voice as long as it is not with lust, and it is forbidden if it is with lust. If a woman is speaking with a non-mahram (definition) man, she should not soften her speech, lest he in whose heart is a disease should covet. It is not permissible for a man to see a picture of a non-mahram woman unless it is in the context of a permissible viewing for direct engagement or via a picture with its regulations.