CheckOut.com is the entertainment-focused e-commerce site backed by Hollywood mogul Michael Ovitz and Richard Wolpert, former president of Disney Online. Sources said MP3.com is also a possible partner.
Details of the relationship were unclear. CheckOut.com would not comment on the deal, and RealNetworks could not be immediately reached.
The move would be RealNetworks' latest effort to bolster its RealJukebox service as a gatekeeper for music downloads and sales.
RealJukebox is an application that converts, or "rips," compact discs and translates them into a digital file format. It also automatically enters information such as song title and artist into a manageable database for music downloaded from the Internet or CDs.
RealNetworks last month released an updated version of RealJukebox and teamed with Warner Music Group, one of the "Big Five" record labels, to offer songs for download from popular artists via the RealJukebox player.
Tomorrow's expected move would take as more major media companies are teaming with Web sites to distribute popular music.
EMusic.com last week struck a deal with Broadcast Music, giving it access to BMI's roster of 3 million songs. And Virgin Entertainment Group last week launched an online service dubbed Virgin JamCast in association with Wavo, whose technology is meant to speed the delivery of online music content.
News.com's Jennifer Balderama contributed to this report.