I think you are stuck with the garmin sourced voices accessible with Web Updater. The 660 is not compatible with the make it yourself type.
The only work around Iknow is if you want to add a new voice to a GPX file, you can make or download a MP3 of the voice and include it in the POI load (make sure the name is the same as the gpx file). Only downside is if you are listening to MP3 music, it will revert to the start of the track when the voice MP3 plays. I listen to talking books in MP3 format, so being blown back to the start of the track when a voice plays is not acceptable, which is why I don't use or recommend this work around. But it may be useful for you if you don't use the MP3 facillity otherwise.