The internet unit of Cox Enterprises Inc has invested $45m in MP3.com. Cox Interactive Media Inc will form a joint venture with San Diego, California-based MP3.com, a distributor of music over the internet in the popular MP3 digital audio compression format. Atlanta, Georgia-based Cox Enterprises Inc president and chief operating officer David Easterly will take a seat on MP3.com’s board of directors. The companies gave no further details and MP3.com executives were unable to comment, as the company is in its mandatory pre-IPO quiet period.