it is so legal....
No, it's illegal. Quake 3 sound files are copyrighted by "id Software". The games source code is open source, but not the game data. Just because a lot of people on the internet use the sound files without regard does not make it legal (a popular example would be the Warcraft III mod DoTA, a lot of Counter Strike 1.6 mods). Luckily, id Software does not get all pissy about it and sue people around the internet like some other companies.
As for the OpenArena sound files, you can find them in one of the pk3 files (just open them with any zip handling archive program like 7-zip). Specifically, pak6-misc.pk3, in the sound/feedback/ directory. Just remember, that OA sound files are licensed under GPL version 2.