NEW YORK (AP) -- Chris Brown and Rihanna are back together, musically speaking. Two songs featuring the singers have hit the Internet. Brown is singing on the remix of Rihanna's track "Birthday Cake," and she's on the remix of his song "Turn Up the Music." Brown tweeted a link to the new version of "Turn Up the Music." Rihanna later retweeted it. She also tweeted about the remix for "Birthday Cake." Brown admitted assaulting then-girlfriend Rihanna at a pre-Grammy Awards party in 2009. He's serving five years of probation for the felony attack. He made his return to the Grammys this year, performing twice and winning best R&B album. Rihanna also performed at the show and picked up a trophy. Brown also tweeted "HAPPY BIRTHDAY" to Rihanna, who turned 24 yesterday.