Adding voiceover without video recompression I have some mp4 video files with pretty poor video quality. I need to add a voiceover translation. I have seen some video software programs that could do the job but unfortunately most of these programs recompress also the video stream, so it becomes worse.

Is there any (preferably, free) software, which would allow me to remix only audio part of my mp4 files and leave the video stream as is, without recompression?
Yes, it's possible. What you need to do is demux and remux. Check out free tools like VirtualDub and Any Video Converter to split the original into separate streams, add what you want using Audacity or similar, then re-multiplex them together using a tool like AviMux. The video can remain unchanged.