If you have output configured to 2 channels / stereo (which is the default unless you changed it or enabled passthrough) and are playing a multi-channel file (eg AC3 or DTS audio) you can adjust the center downmix level (eg make center channel/voices louder) during playback:
push the OK button on the remote, then choose settings (gear icon)->Audio settings and adjust "Downmix: Center Mix Level" - +5dB usually is a good choice for action movies with too loud sound effects.
If the file has 2 channel audio then there's nothing you can do.
If the file is multi-channel and you output multi-channel or have passthrough activated then adjust the center level on your amp/AVR.
so long,
Hias