The SSA/VIOT plug-in does not just add the voice capability, it adds the ability to search and play your music library by artist, album, or genre (in addition to playlist, which is already supported). If you have hundreds of albums, that functionality is well worth the $$.
The 910 Kegs did come with VIOT. If the previous owner actually used the Keg, and enabled the VIOT plug-in, it may still work. If he did not wipe the DMS, the plug-in directory should still be on the phtsys partition of the DMS.
To see if it works, you need a speech engine. You can download the Microsoft Speech engine (
http://download.microsoft.com/download/speechSDK/SDK/5.1/WXP/EN-US/speechsdk51.exe) and install on your computer. Then make sure voice prompts are enabled in PMM, and select one of the available voices. The MS voices are nowhere near as good as the AT&T voices, but better than nothing. Then follow the "how it works" instructions and see if it works.
If it does not work, then not getting the CD including the AT&T voices, as well as the plug-in license, may have been a costly mistake. Unfortunately, I doubt very much that Phatnoise is just going to part with a VIOT license on the basis of your second-hand purchase. If the ebay seller did not tell you beforehand that the CD was missing, give him a negative rating until he comes up with it.