This is the 713th most frequent Norwegian word.
hørte
"Heard" (past tense of "hear").
In this sentence, 'hørte' is used in its past tense to describe perceiving a sound with one's ears. It directly refers to the act of hearing a mysterious sound.
Hun hørte et mystisk lyd utenfor vinduet.
She heard a mysterious sound outside the window.
Here, 'hørte' is used to indicate that information was received or learned via auditory means, again using the past tense to place the action in the past.
Vi hørte at de allerede hadde dratt.
We heard that they had already left.
The verb 'hørte' is in its past tense form and is used to describe the action of listening. The phrase 'hørte på' specifically indicates the act of listening to something, in this case, music.