Are named (in themes slang) the "navigation sounds" and as said are inside system_plugin.rco
There are more sounds spreaded around in other .rco's though, see this table
http://www.psdevwiki.com/ps3/Template:RCOXML_contents_PS3
Most of them are just "clicks" very short (in timelenght, like half a second each) and the firmware doesnt admits longer sounds, if you replace them with longer sounds doesnt plays
The firmware is very picky with this sounds, so for experiments start your tests by building a theme.p3t until you are 100% sure that all sounds plays fine (and you are happy with them because what uses to happen is the sounds plays fine and sounded fine in pc but when you play them in ps3 looks like you made a shitty selection of sounds and you will want to make new ones, lol)
In this wiki page there is some info about sounds and themes in general
http://www.psdevwiki.com/ps3/PlayStation_3_Theme_(P3T)
When you are ready (your custom sounds plays fine inside a theme) you should use rcomage to rebuild the .rco
The correct way to do it (there are others, but this one is the more safer and simple) is by extracting all the .rco contents in "raw" (dont choose any format conversion in the gui)
Keep this files at safe and dont modify them
Now from command line... you need to use the internal WAV2VAG conversor integrated inside rcomage
http://www.psdevwiki.com/ps3/Rcomage
Code:
rcomage.exe vagenc your.wav output.ch1 output.ch2
The .wav file as said needs to be shorter than half a second (or so), there are a lot of files you can find in google by searching for "click + sound + wav"
Also, it needs to be stereo (the output.ch1 and output.ch2 are the left and right channels separated)
Is ok to use a mono .wav though... the output will be only 1 file... what you need to do then is to copypaste it with another name in your pc and then you have the 2 channels (in this case the left and right channels will be exactlly the same sound data but is ok)
The other alternative to create .vag sounds is with the program MFaudio (used for PS2) but there is no really need for it, rcomage does that conversion fiine
And thats all, when you have your .vag files tested in a theme and ready... just replace them in the files extracted before from system_plugin.rco and use rcomageGUI to rebuild the .rco with the "compile" option
---------------
Also btw, fyi
@tarik the reason why this mod cant be made just by replacing a file is because system_plugin.rco is firmware dependant, it changes sometimes when sony releases new firmwares, and this means you cant use an old version of it in a new firmware (there is a semibrick risk, or simply some stuff could stop working, or icons could become invisible)
If you find someone in internet that published a modifyed .rco that matches with your firmware then is just a matter of overwriting it in your PS3... but this kind of mods are not mantained along time, soon or later the files published becomes obsolete because at some point lost the compatibility with newer firmwares