The OBS help has a list of post production tools that could be used for a variety of different tasks:
OBS Knowledge Base. Useful tools that you can use after recording
obsproject.com
If your video contains a mix of too loud game sound and a drowned voice in the same audio track, it's not really possible to change the volume of only one part.
If the voice is missing completely, and you recorded game audio and voice audio to different audio tracks, you need to mix both tracks with a postprocessing software. Media players will only play back one single track, which you can switch in the audio setup of the media player, and if you actually recorded game audio and voice audio to different tracks, you will be able to hear either of them, but not both at the same time.