why is higgens not interested in recording the girl's voice?

Higgins isn't interested in recording Eliza's voice because he already has all of the records he needs that include her dialect.

"Why, this is the girl I jotted down last night. She's no use: I've got all the records I want of the Lisson Grove lingo; and I'm not going to waste another cylinder on it. [To the girl] Be off with you: I don't want you."