1) extract the audio from the *.mkv file
2) convert the audio (whatever the form is) to *.wav
3) use any *.wav editor to increase the overall volume (personally i like Nero Wav Editor best)
4) save the edited *wav as *.mp3
5 mux back the audio into your *.mkv file
*** you can discard the original audio or just inject the newly made *.mp3 into the *.mkv file, making it multi-audio