Mitsubishi Ki-51 "Sonia" In Action in China 1944, 九九軍偵 一号作戦, 長沙衡陽作戦 - video

From April 1944 until December of the same year, the IJA launched "Operation Ichi-Go" with the main goal to capture air bases in southeast China from which American bombers were attacking the Japanese homeland and shipping. A part of this operation was the Campaign of Changsha-Hengyang during which the attached video, dated July 6, 1944, from the NHK collection was shot.
It features a flight of Mitsubishi Ki-51 Assault Aircraft flying over the Chinese mainland with the narator explaining that they take part in the Battle of Changsha. 
Three "Sonia" equipped IJAAf units took part in this operation; the 6th Sentai, the 44th Sentai and the 54th Dokuritsu Chutai (Independent Company).

A tail marking is no visible on this aircraft so we can't say with certainty which unit it belonged to. Prior to and in preparation for the operation, the 5th Air Army responsible for the air operations at that time and place, was receiving 50 aircraft per month and the flight crews in China had to go to Japan to fly them back to their Chinese bases. So, perhaps what we see in the video are newly arrived aircraft on a training flight over China just before the begining of the "Ichi-Go Operation".

The 54th Sentai used a small hinomaru for its tail marking, so perhaps the "Sonias" in the video belonged to that unit.

The NHK collection includes a second video featuring Mitsubishi Ki-51 "Sonia" active during the "Operation Ichi-Go", and more specifically, the Changsha-Hengyang Campaign, and this one is dated August 3, 1944.

The three "Sonia" equipped IJAAf units, the 6th Sentai with 18 Ki-51s, the 44th Sentai with 11 Ki-51s and 14 Tachikawa Ki-36s "Ida" as well as the 54th Dokuritsu Chutai (Independent Company) with about a dozen aircraft (mixed Ki-51s and Ki-36s?), formed the Provisional Air Units within the 5th Air Army. They were based in Pailochi airfield (present-day Bailuqi?), northeast of Yueyang, and Xiangtan Airfield and 

"they were assigned the mission of cooperating directly with the Eleventh Army. They did cooperate closely with the front line divisions in locating the enemy and also acted as liaison between the different ground units during the intervals between enemy attacks.

At first the Provisional Air Unit used Puchi and Pailochi airfields but, with the development of the operation, it advanced to Changsha and Xiangtan. As soon as the Army occupied Hengyang airfield, the Air Unit, disregarding the strong enemy resistance still in the streets of Hengyang and the fact that the field was still under enemy fire, landed and closely cooperated with the Army in the battle of Hengyang.

At all times, this air unit advanced to airfields as close to the front line as possible and closely cooperated with ground operations." 

 

In the clip the Ki-51s do not seem to have any clear unit markings but small hinomarus on the tail are sometimes barely visible. If this is true then this would mean that they belong to the 44th Sentai.

In the background we can probably see Dongting Lake and this would mean that the planes in the clip were operating from Pailochi Airfield.