Here's why:
* Intransitive verbs do not take a direct object. They express a complete action or state of being without needing to act upon something else.
* Transitive verbs require a direct object to receive the action of the verb.
In this sentence, "sounds" is the verb and describes the state of the CD player. It's not acting upon anything directly. There is no object receiving the action of "sounding."
Let's compare it to a transitive example:
* "She closed the door." Here, "closed" is the transitive verb and "door" is the direct object receiving the action.