Weird Video / Audio Problems When I Edit

[ialwayshaveproblems]

Hello,

I use Sony Vegas 17 for my editing now, after previously using Filmora. I realized that with Sony Vegas that I need to convert my MKV files to MP4 files to put it in the program. I used VLC to convert, and now I have an MP4 file.

Here is where things get weird.

If I open the MP4 file in Microsoft's media player, I get video but no audio. If I put the MP4 file into Sony Vegas, I get audio but no video. However, if I put it into VLC, I get both audio and video.

Does anyone know how I can fix this so I can have video and audio for my editing? Much appreciated to anyone who can. If you have any questions about certain things / steps I took, please ask away and I'll be quick to answer. I need to edit a video that I have a deadline for, so I'm hoping for a quick solution.

[ProWo]

Remux (Multiplex) your MKV with my smart FFMpeg gui, select mp4 as container. Identify the track numbers first (Show tracks). Load your MKV as video source, then load it as Audio source too. Select the right track numbers. Is done in seconds.
https://forum.videohelp.com/threads/395425-New-small-GUI-for-FFmpeg