Hello @lumiville and welcome to the forum 
The audio sync problem is a long standing issue but it doesn’t happen for everybody and its causes are a mystery. There has been recent effort to improve this and it’s hoped that version 5.2, when it’s released will improve audio synching.
It’s discussed in this topic but only available as an appimage for Linux at the moment:
Animation Audio: Development Test (Linux AppImage)
As for the rendering failure,
Does that mean you were rendering from part way into the animation, i.e not starting at frame-0?
If so, this has been seen by other people and has been formally reported as a bug here:
https://bugs.kde.org/show_bug.cgi?id=459993
It hasn’t been fixed yet.
The person who reported the bug does mention a workaround but I’m not sure that I understand what their description of it means.
My workaround for it would be to Render out to an image sequence with sequence frame numbering starting at zero to a folder.
Then create a new file of the same canvas size, then File → Import animation frames, from the rendered image sequence folder.
Then Render it all out to video.
Please reply to say if that would work for you.