As general trouble-shooting, you should run Disk Utility (in Applications/Utilities). Select the "Macintosh HD" (your startup volume) in the Disk Utility sidebar. Over to the right, go to the First Aid tab. Use Verify Disk. This will check for data corruption issues on your startup volume.
Make sure your startup volume is not getting to close to being full.
I would create a new user account in System Preferences Accounts pane. It can be admin or standard.
Log out (to the login screen) and log in to the new user account (don't use Fast User Switching from the menu bar).
Try doing some text-to-speech in the new user account. If it works better in the new account, that means the problem is something isolated to your normal user account. It may be a background process that runs in your normal account that is interfering with text to speech, or it may be a corrupted user preferences setting. If the problem recurs in the new user account, then the problem is a system wide issue. That will help narrow down your trouble-shooting.
Try some hardware resets first. This is reseting PRAM
and resetting SMC (power management)
If that does not help, then it is more likely to be a software issue affecting the overall OS. It might be a third-party system extension that is causing a conflict, or some system component may have become corrupted since the original installation. System extensions are installed here
but it is often difficult to see anything that is obviously wrong. If you see anything listed that is obviously for hardware or software that you no longer use, you can try removing it.
Short of doing a complete re-install of Mac OS X, it may help to apply the 10.6.8 "combo" update manually. This will re-install all the system components that have been updated since the initial 10.6 release. It can sometimes fix mysterious problems that are difficult to diagnose precisely. This is the combo updater for 10.6.8.
NOTE: Before doing a system update, you should have a good backup. If you have been using Time Machine, that will serve as your backup. Things can go wrong during system updates, and you don't want to end up with an unbootable system, with no backup.
I tried resetting PRAM and resetting the SMC but that hasnt helped me either. I have tried to apple the 10.6.8 combo update manually once and I am trying it again as I later found a bunch of old third party extensions which I deleted after I had first tried it. Another weird thing I noticed was that only the "Alex" voice is is glitching up, the rest of voices work fine, maybe that is a clue as to what is wrong here?
Another weird thing I noticed was that only the "Alex" voice is is glitching up
I think that's a BIG clue. After looking around, I found that the "voices" are stored here
<your startup volume>/System/Library/Speech/Voices/
Actually, I'm using Lion, not Snow Leopard, so if you are using 10.6.8, it may be different, because there were some changes to Speech with Lion (with higher-quality voices).
However, what I see there are folders corresponding to the voices. So, if Alex is not working right, but the other voices are fine, maybe something in the "Alex" voice folder is corrupted.
Do you use Time Machine? If you have an old backup state in your Time Machine archive, that goes back to when you installed Snow Leopard (or when this problem did not exist), you can try copying the (current) "Alex" voice folder to another location, deleting it from that location, and restoring it from Time Machine (before you had this problem).