"Regarding the first debate, there were issues regarding Donald Trump's audio that affected the sound level in the debate hall," the commission said in a statement. No other details were immediately made available.
The statement provided no evidence that the audio issue affected the television audience of more than 80 million people.
To CNN producers inside the debate hall, it sounded as if Trump's audio was at a lower level at the beginning of several answers, perhaps as a result of him standing too far away from the microphone attached to his lectern. During the course of those answers, it appeared as if someone had increased the audio level, making Trump sound louder in the hall.
His audio level never seemed to stay low for an entire answer. The audio issue appears to have been limited to what was heard by the audience inside the debate hall, and the variations in audio were not heard by viewers watching the debate on television.
The PA system in the auditorium and the TV audio were operated separately.
Trump had complained earlier in the week that his microphone wasn't functioning properly at Monday's debate, though Friday's statement by the commission said nothing about a microphone problem.
"And they also had, gave me a defective mic. Did you notice that? My mic was defective within the room," he told a group of reporters afterward. "No, but I wonder, was that on purpose? Was that on purpose? But I had a mic that wasn't worked properly, with, working properly within the room."
Speaking at a rally in Novi, Michigan, Friday afternoon, Trump said the commission's announcement vindicated him.
"It was just as I was saying," Trump declared. "Working that mic was a hell of a lot more difficult than working crooked Hillary Clinton."
He added: "I wonder why it was bad. I wonder why it was bad," though he did not speculate further.
In a New York Times interview
released Friday night, Trump said he "spent 50% of my thought process" dealing with the issue. He blamed any problems he had at the debate -- his performance was widely panned -- on the technical problems.
Trump's Democratic challenger, Hillary Clinton, mocked Trump the day after the debate for complaining about his mic.
"Anybody who complains about the microphone is not having a good night," Clinton told reporters.