I'm not familiar with Macs, but I can hazard a guess: look in your settings for ... audio out? digital out? See if it's hard set to 96. See if there's a setting in ...ac3 filter? Are you using that? Looks like it's under the main tab, "output format", set to "AS IS"? Looks like there might be a couple of settings there, if you're using it:
I would love to help, but really need to be looking over your shoulder, alas. Look at the settings under SPDIF on the same tab as well, unclick the "restrict" rate if you're using it.
EDIT: Assuming you're not using ac3 filter, it looks like the settings you need to be looking at are your audio settings. It should look something like this:
...only you want to set your output device to your SPDIF. If you can do that and take a screen shot, I'm sure I or someone here can tell you what to look for next, assuming you haven't figured it out yourself by that point.