Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
The speech of the dead in a dream is truth and honesty, because they are in the abode of truth. So whatever they say is correct. Whoever sees himself talking to a dead person will have a long life.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
Whoever sees a dead person talking to him, there will be reconciliation between him and people after a dispute.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
The speech of the dead indicates an admonition for the dreamer. If they command him to do something, he should do it if it is good.