Back when ut99 first came out there was like everybody and their 3rd cousin had a website for the game. A lot of good stuff, but some it sucked too... though one that truly ruled was Ram2000 which specialized in voicepacks for the game. Sadly it's gone now and I haven't found any other site anywhere near as good with VPs since then.
Standard voicepack is 3 files: .int, .uax, .u but after awhile some people started ditching the .uax and shoving all the sound files directly into the .u file. That said, if you're gonna go with stuff you have on your own system that's not readily availble to others (stuff you collected, not stuff you made) it can be hard to tell which voicepacks require what files.
There's probably an app for that.
papercoffee asked what voicepacks I have the other day, so here's a quick list:
(http://ic.pics.livejournal.com/evilgrins/3189901/3315550/3315550_original.png) (http://ic.pics.livejournal.com/evilgrins/3189901/3315719/3315719_original.png) (http://ic.pics.livejournal.com/evilgrins/3189901/3315991/3315991_original.png) (http://ic.pics.livejournal.com/evilgrins/3189901/3316283/3316283_original.png) (http://ic.pics.livejournal.com/evilgrins/3189901/3316708/3316708_original.png)
I may have more, specific to certain models, but I think this is most of them.
That's a lot of voicepacks.
Hey I hope you back up your hard drive, that's a bazillion UT things you'd lose otherwise.
I've actually had my drive crash something fierce a few times over the past couple decades... but fortunately I know a guy who can fully recover what I thought had been lost.