I was finally able to burn a dvd with a video src that had video and audio and add an extra audio src for audio background as i wanted to.
It was fixed by uninstalling Nero 220.127.116.11 and installing 18.104.22.168.
Partly thanks to the help of some people who responded to my questions. Wanted to say thank you first of all.
I still have a problem though.
The transcoding takes 45 seconds in NeroVision Express.
But takes me 3 minutes in the api.
My Mpgs are supposed to be dvd compliant already.
Is the smart encoding working correctly?
Removing the audio background and transcoding the videos with their original sound does not change the speed.