If your audio drops out from a specific point in the video until the end, you most likely have a corrupt video that needs to be re-exported from your editing program. The audio will drop out right where the video file becomes corrupt.

However, if you experience an issue where the audio drops out only during playback of specific sections of the video, or just the vocal track can't be heard while music plays, you are most likely experiencing the audio phenomenon known as phase cancellation.

This would be caused by the method in which you recorded the audio for those segments, by placing a mono source (microphone) into stereo right-and-left channels with opposite phases. Since the content in both the right and left channel are identical except with opposite phases, when they are combined back into mono playback through certain devices (typically older mobile devices, possibly a laptop), the microphone audio is eliminated through the audio phenomenon of phase cancellation where the waves of multiple tracks work against each other to eliminate the frequencies.
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​In order to fix the issue, you will want to select the audio tracks in your timeline that were recorded in this manner and apply a "Fill Left" or "Fill Right" audio effect. This will synchronize the audio phases and prevent the phase cancellation issue. Once you correct the issue, you will need to re-export and use the Replace Video feature.