Does it constitute disbelief to contemplate matters of disbelief, the apostasy of one of the spouses, and the change in the ruling on listening to music, and what is the effect of that on the marital relationship?

What you experience in terms of whispers does not cause apostasy. The most effective treatment for whispers is complete avoidance of yielding to Satan in thinking about this matter, along with seeking help through the remembrance of Allah and supplicating to Him. Divorce is in the hands of the man, not the woman. Whispers about divorce do not entail a ruling and should not be paid attention to.