Recording my own voice was easy, but it didn't sound very good as I only have the cheap mic that came with the computer.

I created a complete set of voice prompts using the ATT Text to Voice link. DVD, Cable TV, Tuner, XM Radio CD, Hello, Good Bye, etc. 
[Link: research.att.com]
I was able to adjust the volume in Creative Media's "Wave Studio" (thanks for the tip).
Testing it was a little more painstaking.
Download the voice prompt, copy it into CCP, download to remote, try it, adjust the volume, save as #2, copy to CCP, download again, try it, adjust volume, you get the picture.

After all that, now I'm not sure if I like it. Between the computer generated voice and the tiny speaker it sounds like a kazoo.
I may like the beeps and short jingles better. 

I have a couple of guifx sounds/voice commands.  I'll try them.

It's fun to mess with.

I'm going to start a thread complaining about the tiny speaker URC puts in the MX-980.
It is a new replacement. It may sound better after the speaker break in period. I'll take some base line measurements and chart the results for 90 days and see. [Link: remotecentral.com]