Microsoft has said it is considering developing a subscription-based online music service.
Online music is increasingly popular
The firm's confirmation of a report on techology news website Cnet sent shares in online music providers RealNetworks and Napster down on Friday.
Shares in Apple, home of both the iPod portable music player and the iTunes online music store, also lost ground.
"We are actively investigating the subscription model," a Microsoft spokeswoman said.
"Once we are ready to talk more, we'll let you know," Christine Andrews, lead product manager in Microsoft's MSN internet division, told Reuters.
Apple shares fell 4.9% on Friday on the Cnet report, while RealNetworks lost more than 3%.
RealNetworks provides the Rhapsody subscription service which allows users to download songs for a monthly fee.
Shares in Napster, which pioneered online music downloads, fell 4%.
Yahoo's shares fell 1.7%. It also provides a music subscription service.