In the message to her longtime friend and label executive, Dan Waite, O'Riordan discussed covering The Cranberries' song "Zombie" with the hard rock band Bad Wolves — saying it sounded "f—ing terribly good," according to the audio of the voicemail published by TMZ Thursday.
The timestamp of the voicemail was 1:12 a.m. in London, just hours before O'Riordan would be found dead in her hotel room around 9 a.m.
"I'm in London... I think it's f—ing awesome," O'Riordan appears to have said to Waite, who set up the collaboration, about the Bad Wolves' track.
"It sounds f—ing terribly good. I think I can come into the studio if you need me," she added, before sending love to Waite's family and hanging up.
The 46-year-old was visiting London to record vocals for the famous Cranberries track. If the voicemail is any indication, she sounded excited to get into the studio.
On the day of her death, Waite told PEOPLE that O'Riordan had left him a voice message "just after midnight" and that she sounded "full of life."
“Dolores left me a voice message just after midnight last night stating how much she loved Bad Wolves’ version of ‘Zombie.’ She was looking forward to seeing me in the studio and recording vocals,” Waite said.
“She sounded full of life, was joking and excited to see me and my wife this week. The news of her passing is devastating and my thoughts are with Don her ex-husband, her children and her mother,” he added.
The inquest into her death was opened days after she died, but was adjourned until April 3 while the coroner awaited toxicology results, according to the BBC.
Police ruled her death non-suspicious, meaning they found no evidence of foul play.
Authorities found counterfeit fentanyl near the rocker's bed, a source told the Santa Monica Observer, and a source in the London Police Department said O'Riordan died of fentanyl poisoning. The source adding that suicide and a deliberate overdose are suspected.
After splitting from her husband of 20 years in 2014, O'Riordan's life took a hard personal turn. The Irish singer spoke publicly about suicide and her bipolar disorder, which she said she was diagnosed with in 2015 and said she'd struggled with the symptoms for years.
In 2014, she told the Belfast Telegraph that she once tried to take her own life. “I tried to overdose last year... I suppose I am meant to stay here for the kids,” she said at the time.1comments
She is survived by three children: Taylor Baxter, 20, and Molly Leigh, 16, and Dakota Rain, 12.
During the interview, the “Zombie” singer said she had to be careful not to keep sleeping tablets around “because if I have a few drinks I’ll take them…. Then you don’t wake up.”