What did Stephano says about the music on the island?

In Act III, Scene 2 of The Tempest, Caliban, Stephano, and Trinculo are drinking alcohol and conversing. Ariel arrives and, while staying out of sight, interjects comments into their conversation. Caliban knows that Ariel is doing this, but does not want the others to know that the sprite is there. As Caliban finishes convincing the sailors to “destroy” Prospero and take his books of magic spells, he asks the others if they would like to resume singing the song that Stephano had recently taught him; he uses a fishing metaphor, “troll the catch,” for singing.
As Stephano begins to sing, Ariel joins in but plays a different tune on their drum and flute (“tabour and pipe”). Stephano immediately notices that it is not the same and asks what tune it is. Trinculo says it is their tune, as played by some unseen entity, “the picture of Nobody.”
Stephano freaks out a bit, demanding that this Nobody appear if he is “a man” or do as he please if he is “a devil.”
To assuage his fears, Caliban then tells him about the island’s delightful magical sounds and music, which will not hurt them.

Sometimes a thousand twangling instruments
Will hum about mine ears, and sometime voices
That, if I then had waked after long sleep,
Will make me sleep again….

Stephano seems convinced, stating that in this “brave kingdom . . . I shall have my music for nothing.” As Ariel continues to play, they decide to follow the drum’s loud sound.
http://www.opensourceshakespeare.org/views/plays/play_view.php?WorkID=tempest&Act=3&Scene=2&Scope=scene

The drunken servant Stephano has set himself up as ruler of the island, with Caliban as his willing slave. Caliban's so desperate to be free of Prospero's yoke that he's prepared to conspire with his new master to kill Prospero and steal the magic books which he uses to consolidate his power.
But Prospero's loyal servant, the ever-faithful Ariel, is listening in to the treacherous conversation of Stephano, Caliban, and Trinculo. And with a pipe and drum he provides a musical accompaniment to the traitors' premature song of victory. Stephano is immediately unnerved by the sound of music:

What is this same? (Act III, Scene ii).

In other words, he wants to know what this strange song is. Stephano's too drunk to realize that it's the melody of the freedom song he'd just started to sing. A line from that song states that "thought is free." But the presence of Ariel is a surreptitious reminder that the would-be conspirators are not really free not all, not with Prospero's loyal servant hanging around, eavesdropping on their conversation.