You have to make your own pk3 file with the same folder structure as in pak0.pk3.
Your main folder will be sound.
So to change the 30 sec sound you have to build up the structure : sound/vo/general/allies/hq_30seconds_a.wav and hq_30seconds_b.wav
(you have to rename your sound twice to be sure it will be played every time - as it is 2 sounds for the same announcement).
And also change it for axis if you want all the players on the server to hear the same sound - are you planning to put in music for the last 30 seonds?
The same goes for weapon sounds : sound/weapons/colt/colt_fire.wav (give the new sound the same filename).
No need to for any scripting.