The singer's fiancee celebrated her 35th birthday on Saturday (19.04.14) as Muse headlined the main stage at the festival in Indio, California.
Earlier in the month, Matt revealed on Twitter that he had laryngitis and instead decided to let the audience sing to the actress at the weekend.
He told the crowd: ''It's my girlfriend's birthday today. Her name is Kate, she's out there somewhere,'' adding, ''I want everyone to sing Happy Birthday to her.''
Rumours surfaced in February that Kate and Matthew, 35, had drifted apart when the blonde beauty was spotted at the BVLGARI's Decades of Glamour party without her engagement ring.
But the couple - who got engaged in 2011 and have two-year-old son Bingham together, as well as raising Kate's 10-year-old son Ryder from a previous relationship - appear to be putting on a united front.
Meanwhile, Matt and his band mates - Dominic Howard and Chris Wolstenholme - will begin working on their seventh album next month and hope to complete it by the end of 2014.
The Guardians return two months after their epic battle against Ronan with their criminal records erased