"The tiniest thing will set them off," a source says. "Charlie has a bad temper. It gets scary."
A source tells us the daughter is the oldest of the two, 5-year-old Sam...
The song apparently has something to do with a Christmas present Sheen bought for Sam. Sheen has told people that Mueller became upset and wanted to know why she and Sheen didn't have a similar song, the source says.
"Charlie says she is really jealous of his daughters, especially after he and Denise finally started getting along," the source says.
The source also says Mueller has become increasingly suspicious of Sheen because of his past substance abuse issues and fondness for call girls.
"Obviously the fight wasn't just about the song," the source says. "Nothing excuses what Charlie may have done, but there's more there."
The source said many around the couple believe that both of them need help. "They've got to think about those two little boys," the source said, referring to Sheen and Mueller's 9-month-old twin sons, Bob and Max. "But yeah, I think it will end in divorce."
Check out our Charlie Sheen & Brooke Mueller: Better Days photo gallery to see when things weren't so chaotic.