'FFMPEG, Increase Volume Without Ruining Subtitles?

I have a video file with very low volume and I am trying to use FFMPEG to increase it:

ffmpeg -i input.mkv -vcodec copy -af "volume=31.9dB" output.mkv

There are hardcoded subtitles for the scenes that are not in English. The code works for the volume but it makes the subtitles black and impossible to read. How can I increase the volume without ruining the subtitles?

ffmpegcolorsvolume


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