Hero Mix is a cancelled music game / music tool that was in development by 7 Studios around 2010, planned to be published by Activision on Xbox 360 (and possibly Playstation 3). As you can assume by its title it was meant to be part of Activision’s Hero series of music games, composed of such games as Guitar Hero, DJ Hero, Band Hero and the cancelled Sing Hero, Dance Hero and Drum Hero.
Activision never announced this game officially, but some screenshots with mock-up UI were found online by fans in 2013. By looking at these we can speculate you would have been able to mix together different songs by playing some kind of rhythm game. Basically it could have followed the same premise of other Hero music games, but with mixing mechanics instead than playing a plastic guitar or DJ set.
In the end the Hero music game market collapsed and Activision just canned all the remaining projects still in development:
“In October 2009, following the completion of DJ Hero, Activision laid off an estimated 30 people from 7 Studios, about half of the team. In February 2011 Activision announced to discontinue all music games series. On 9th February 2011 staff members confirmed that 7 Studios had been shut down.”