Hi. I played Wing Commander 2 back in the bad old days when speech acting was first becoming widespread. I had to disable the sound in the game in order to get subtitles. Is there a similar option for the GOG version? I see someone mentioned in another post something about hitting alt+o but it didn't mention which games in the series that applies to and someone in the same thread said it wasn't possible at all.

I think WC2 only has speech in the intro and during the combat sequences. I don't think the cutscenes have speech, at least if they do, I can't get them to work.

Vanilla WC2 back in the day only had speech in the intro. You needed to install the speech pack add on to get more speech. The Addon gave you speech in space combat and also in selected cutscenes throughout the game, but not all of them. The GOG version includes the speech pack.

If you figure out the code for adlib I suppose changing wc2.cfg would be one way to disable speech since adlib doesn't support speech.

Don't worry, purchase it :)
As far as I know, there is no way to change subtitle settings in WC2 in-game. However, changing the DOSBox emulation setting is really easy and 100% working (just tested).
After the install, open the dosboxWC2.conf file in WC2 directory and change
sbtype=sb16 to sbtype=none
and
oplmode=auto to oplmode=opl2

After this the game will start without detecting Soudblaster (thus with subtitles) yet still emulating AdLib music.