Unfortunately I can't hear the voices at all over the general sounds of combat in a raid. I tank, and there's so much general melee noise up by the boss that the addon is nearly useless. I would really love for this to be useful, but I can't figure out if there's a way to either raise the voice volume, or to somehow lower just the combat sound level.

I think it would be very useful if you could export the ogg files with a much higher output level or something along those lines. I believe Audacity can batch update a whole bunch of files at once and amplify them. https://forum.xda-developers.com/showpost.php?s=edfa5b35d2b9dde60174f609e21dd165&p=39857842&postcount=2

In Audacity you can also use File -> Edit Chains to make a chain for ogg amplification. I've done this as a test locally with a 10db boost, and it did update every ogg file to be louder. I haven't been able to test it in-game yet though to see if it's loud enough.

Under System/Sound, it might be possible to increase the Master volume while lowering the Sound volume. The voice files are played on the Master channel.

Do the combat sounds not play over the master channel?

I think the game sounds play over the SFX channel, which is just marked "Sound" in the settings.

If by combat sounds you mean game sound effects then no.