Duo
Duo-R
Duo-RX
The audio capacitors in the first-generation Duos leak and stop producing sound over time. It's not a terribly difficult fix (just replace the caps or have someone who can solder do it for you), but it's best to be aware of what you're getting. All American Duos fall into this category; the -R and -RX were released in Japan only.
From what I understand the Duo-RX is most resistant to this type of decay due to a revised mainboard layout, although I've never owned an RX myself.