Exporting AAC 5.1 channel audio Help in windows

I’m trying to export AAC 5.1 channel audio in Audacity but not able to export more than 19 minutes. My audio project is longer than 2 hours. I’m using this by external program by using these commands.
ffmpeg -i - -c:a aac -b:a 256k “%f”
ffmpeg -i - -c:a aac -q:a 5 “%f”
ffmpeg -i - -strict experimental -c:a aac -b:a 256k “%f”
ffmpeg -i - -strict experimental -c:a aac -q:a 8 “%f”

Basically I’m trying to replace third track of one AC3 5.1ch audio with one mp3 mono track. And after it, I’m exporting. I tried on many times taking different different samples and on different PC but not able to get AAC5.1 ch audio but, at the same time it is well in exporting AC3 5.1 channel sound. During AAC5.1 export It throws this error.

[pcm_s16le @ 00000000004c8620] Multiple frames in a packet.
[pcm_s16le @ 00000000004c8620] Invalid PCM packet, data has size 8 but at least a size of 12 was expected
Error while decoding stream #0:0: Invalid data found when processing input

These types of error do not arise when export shorter audio length less than 10 minutes. Plz help me.
It will help others too.