Nicole McKibben told a Fatal Accident Inquiry into his death that over several months she heard him screaming.
His mummified body was found in his litter-strewn cot at the home he shared with mother Kimberley in Paisley in March 2010.
She was found guilty of neglecting and murdering him and was jailed for life in 2012. But last year she was released on appeal after a judge ridiculed medical evidence from her trial.
The development prompted the Crown Office to call for a Fatal Accident Inquiry to try and establish how he died.
Ms McKibben wiped away tears as she told Paisley Sheriff Court: "I just thought he sounded distressed. I wish now I had done something about it.
"It's something I'll regret for the rest of my life."
The witness said that everything was normal when Ms Hainey and Declan moved in at the end of 2008.
She said she would hear noises coming from their flat all the time up until around the time of Declan's first birthday, in April 2009.
But by summer and autumn 2009, things were becoming different.
She said: "There were things that started to change - the noises became more frequent and louder.
"Kim would put the TV on for long, long periods with the volume up very loudly.
Ms McKibben said the TV could be blaring for up to five hours at a time and that Declan's crying changed.
She explained: "The crying became more and more frequent. I would hear him crying every day.
"There was more and more crying - it started to make me question what was going on.
"He was crying continually for months."
The inquiry, before Sheriff Ruth Anderson, continues next week.