Sorry for the late response, but I wanted to offer some help with your Sony Vegas issue. Have you tried using tsmuxer to convert your MKV files into an m2ts format? It might work better with Sony Vegas. As for the rendering issues and pixelation, they could be related to your project settings or output settings. Make sure you have the right settings for resolution and bitrate. By the way, I came across this cool automatic video editor online that has a feature to easily add background music to videos https://www.movavi.com/support/how-to/how-to-add-background-music-to-video.html. It might be worth checking out! I hope this helps.