Audio - ffmpeg produces high-pitched voices

I have a big video file and I'm using ffmpeg to get a smaller one. I work with the 2-pass ratecontrol and I'm using this simple command line

ffmpeg -y -i file.mkv -c:v lix264 -b:v 900k -ac 2 -c:a aac -q:a 0.8 -pass 1 -f mp4 NUL && ffmpeg -i file.mkv -c:v lix264 -b:v 900k -ac 2 -c:a aac -q:a 0.8 -pass 2 out.mp4

Well, my problem is the following: the original video has 5.1 DTS audio. Mine has stereo but nearly all the dialogues sound high pitched. How can I fix this? Another video I have created doesn't have this problem (I used the same command line).

Also I have read that qaac does a better job with encoding. Is there any way I can use qaac with ffmpeg for the audio?