keeping the bitrate the same for ffmpeg commands audio

So my question is whether anyone knows some more specific commands I could use to get my desired constant audio bitrate out for acc codec?

I just loaded a video in VirtualDub2 and using external encoders option where I can set commandline arguments.I just selected some video frames to encode from the beginning of the video and as result I got first a matching audio bitrate = good.Now I did selected some frames from the end of the video and did encode it and in this second file I got a audio bitrate which is higher.Ĥ4100Hz 128 kb/s tot (2 chnls) <- of first video partĤ4100Hz 129 kb/s tot (2 chnls) <- of second video part i -c:a libfdk_aac -b:a 128k -f mp4 -y '%(tempaudiofile)' I found a new problem using FFmpeg to encode some video / audio streams.My goal was it to encode some diffrent parts of one video out using same audio bitrate but as result I have the parts out with diffrent bitrates what means I can not merge this parts together but for this I need exact same audio bitrate (only for audio needed).Thats a problem so far.I tried already all diffrent ffmpeg commands without success.I am using audio codec aac (nativ).On internet I found some infos about using libfdk_aac instead of acc what I tried too (Internet I found a static compiled build version what has this codec included which is not present in normal builds).I tried this one now and it still dosent match.

Keeping the bitrate the same for ffmpeg commands audio