Obtained: During the Chapter 5 Intermission, speak with Ayaha in Fortuna Office.
After the cutscene, head on over to Daiba Studio and interact with the girl hunched over near the map transition. She is wearing a white long-sleeve top with a plaid green skirt. After you speak with the girl, head back to Ayaha to report the news then head on over to Daiba Studio’s Idolasphere to search for the thief.
Once in the Idolasphere, head through the door on the left to arrive in the Monitor Room. Head in through the door under the monitors and look at your map. Make your way towards the little red boss head to find the glowing Mirage.
Head up to the Mirage to trigger a fight with a Regal Knight (level 55) and two Red Shamans (level 55). Defeat them all to trigger a brief dialog scene.
Exit the Idolasphere and check on Tamae Manai (previously hunched over girl) in Daiba Studio.
After you confirm everything is ok with Manai, travel to Fortuna Office and speak with Ayaha to complete the request.
Reward: Clap Track