You will see a .caf file in there called Voicemail.caf. You need to replace this .caf file with your own .caf file. In order for you to get your .mp3 to a .caf file, you will need to do a little conversion, which ironically, I just got finished doing after reading a post in my thread on changing your text message sounds. Here's the quote:
Renaming alone will not do it with an MP3. conversion is necessary.
CAF is a renamed AIFF file (*.aif). AIFF is an uncompressed file (like a .WAV file).
iTunes can convert any file imported into an AIFF if you go into preferences/advanced/importing and select the AIFF encoder. Then just rename the file to the proper *.caf file you want to replace and SSH it to the phone.