You can do all this in iTunes

put your audio file in iTunes,
right click>get info>options
then choose the start and stop time,
then click okay
then right click on the audio file you modified> click "Create AIFF Version"
Then once it has converted it, locate the file, and change the extensions
from .aiff to .caf
If using windows, make sure under "folder options" you have the setting set to viewing the file extensions.
Thats all there is to it
If your iTunes doesnt show the "Create AIFF Version" then let me know and ill let ya know how to get this option, you just need to fiddle round with the settings to get it

. I cant remember the exact way to do it atm.
Once you have the sounds you want, simply SSH into your phone with WinSCP and replace the originals (BUT REMEMBER TO BACK UP YOUR ORIGINALS!!!!!!! just rename the originals to soundBACKUP.caf etc). When replaceing the sounds make sure you rename them with the EXACT same name, otherwise it wont work.
You can use winterboard to customize system sounds to

place all your system sounds in /private/var/stash/Themes.XXXXXX/theme name.theme/UISounds
and rename them as
beep-beep.caf
lock.caf
mail-sent.caf
new-mail.caf
SentMessage.caf
sms-received1.caf
Tock.caf
unlock.caf
I think thats all these is to it
