I had the same problem, but found a solution that I haven’t seen anyone here suggest.
On a video I have three audio tracks, one stores in-game audio, one stores my voice recording and one stores music. But when I render the video, it only includes the in-game audio.
The solution was to reduce the audio quality from 48,000Hz (default) to the quality of the lowest Hz sound in your project. In my case this was 44,100Hz.
Now it was trying to render the sounds at a quality that that all sounds in my project had, and it worked!