The transcript from the cockpit voice recorder shows the pilot and copilot noticed a steep loss of altitude about one minute before the crash, TV Globo reported in its main newscast Jornal Nacional on Wednesday evening, citing unnamed people working on the investigation.
TV Globo did not release the audio nor the transcript.
According to TV Globo, the transcript covers about two hours of audio recording, including a question from the copilot to the pilot about what was going on, and saying the plane needed "more power" to be stabilized.
The plane, an ATR-72 turboprop from local airline Voepass, was bound for Sao Paulo from Cascavel, in the southern state of Parana, and crashed at around 1:30 p.m. (1630 GMT) in the town of Vinhedo, some 80 km (50 miles) northwest of Sao Paulo.
The crash killed all aboard but no one on the ground was hurt.
Video shared on social media right after the accident showed the ATR-72 aircraft spinning out of control as it plunged down behind a cluster of trees near houses, followed by a large plume of black smoke.
The pilots did not report an emergency or adverse weather conditions, the Brazilian air force said on Friday in a statement.
TV Globo said that, according to people investigating the crash, analysis of the audio alone will not make it possible for now to determine the cause of the crash.
TV Globo also said the authorities did not identify any characteristic sounds, such as from fire, electrical collapse or engine breakdowns, although the audio was said to be difficult to listen to.
A potential icing issue on the plane's wing was neither discarded nor confirmed by authorities, TV Globo said.
Videos of the crash analyzed by aviation experts led some to speculate that ice had built up on the plane.
Brazil's aviation accident investigation center, Cenipa, could not immediately respond to a Reuters request for comment about the TV Globo report outside of regular business hours.
