All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If these CDs are owned by a professor and he does not permit anyone to listen to them, then it is impermissible for anyone to listen to them without his permission. It is possible that he does so in order to force the students to buy them and he is entitled to do so.
‘Ali ibn Zayd narrated on the authority of his cousin that the Prophet, sallallaahu ‘alayhi wa sallam, said: “A Muslim's property is inviolable unless he willingly gives it.” [Ahmad]
In a Marfoo’ Hadeeth, Asmar ibn Mudharris narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever possesses a permissible thing before others then it is his own.” [Adh-Dhiyaa’: Saheeh]
But if you mean that he does not permit the buyer to allow others to listen to these CDs, then this is impermissible because it is a sale condition that was forbidden by the Messenger of Allaah, sallallaahu ‘alayhi wa sallam. The famous opinion is that the transaction is revoked as long as the seller insists on his condition; otherwise, the transaction is valid.
Ibn Rushd said: “Transactions which include conditions, as when a person sells a commodity on condition that the buyer does not sell it or gift it, or when one sells a bondswoman on the condition that the buyer takes her as the mother of his child so that she would be automatically freed after his death, or that he does not take her out of the country, or that he does not get her pregnant, or that he does not make her travel by sea, as well as other conditions which restrict the buyer's behavior. When such transactions take place, the scholars provided different views in this respect. Some of them are of the view that such a transaction is revoked if the seller insists on his condition. Otherwise, the transaction is valid. This is the famous opinion in the Maaliki school of Jurisprudence.”
Allaah Knows best.