Headphone maker Bose is being sued by a customer who claims the firm has gathered data about his listening habits without his permission.
Chicago resident Kyle Zak claims Bose's app scoops up data which is then sold to firms to use for targeting adverts.
Mr Zak wants the court to grant an injunction that stops Bose grabbing data about audio preferences and is seeking $5m (£3.9m) in damages.
Bose said it planned to fight the allegations made against it.
"People put headphones on their head because they think it's private, but they can be giving out information they don't want to share," Christopher Dore, a lawyer representing Mr Zak, told the Reuters news agency.
Mr Dore works for law firm Edelson PC which specialises in cases revolving around data privacy.
Legal papers filed by Edelson said Mr Zak downloaded the Bose Connect app soon after buying a pair of QuietComfort 35 headphones. He provided basic information to sign up for the app that lets users control what they listen to via their smartphone.
Soon after, alleges the lawsuit, he noticed that it was logging far more data about his audio choices than he expected.
The suit claims that similar data is taken from users of other Bose gadgets including the SoundSport Wireless, Sound Sport Pulse Wireless, QuietControl 30, SoundLink Around-Ear Wireless Headphones II, and SoundLink Color II.
Mr Dore said the sign-up process for the app gave no hint about how much data Bose gathered nor what it planned to do with it.
What people listened to gave an "incredible amount of insight" into someone's personal life, religious and political views, he added.
A Bose spokesman said "nothing" was more important to the firm than the trust of its customers. He said the company was sending messages to customers about the lawsuit that will read. "In the Bose Connect App, we don't wiretap your communications, we don't sell your information, and we don't use anything we collect to identify you - or anyone else - by name."