Does the Ogg support in AGS just include Vorbis, or does it have Speex as well?
I've never heard of Speex, so I would suspect not.
Speex is a lossy speech compression format, giving better compression for speech than Vorbis, but its from the same people: http://www.speex.org/
Ogg is a wrapper for different kinds of format, and if you use Ogg's own player (as it says in the docs), then it might just work anyway. Do you always build in the latest version of their player when you release a new AGS?
If they differ only in the encoding phrase, it may be possible to play them with the same player.
BTW, why don't you try it first? Try to stuck a Speex encoded sample into a test game and see if AGS can play it. ;)
Why not give it a go and see, indeed. No, I don't update the ogg player every release unless there's a good reason to - better the devil you know.
But yeah, if they compress to the same format it may well work anyway, so try it out.