Voice notes are recorded as .amr files.
AMR File Extension - Open .AMR files
There should be some converter programs that will convert them to mp3s. I've never tried it myself. I found this on a Google search.
Mobile AMR converter free download. The Mobile AMR converter uses Sony Ericsson AMR converter to convert AMR files (Adaptive Multi-Rate Codec, *.amr) to wave files (*.wav) and vice-versa...
There might be others found if you search on "convert amr to mp3".
If you could convert the the amr file to mp3, you could use it as a ringtone.