Theories/Speculation
Unfortunately, developer Red Octane has never released any peripheral to connect to the expansion port on the PS3 Les Paul guitar. It was rumored that the expansion port would connect to a wired effects pedal. The Guitar Hero V guitar has no expansion port, indicating that this idea may no longer be in consideration.
Appearance
The expansion port looks like a telephone jack, which led some to believe that it could be used for voice communication through a headset. The instruction manual, however, warns gamers not to experiment with the port, cautioning that you should
"not connect a telephone line to the Expansion Port. Doing so may result in damage to the controller or cause a fire."
Similar Ports
Red Octane also developed the X-Plorer guitar for the Xbox 360 version of Guitar Hero: II. The X-Plorer has a similar expansion port; this expansion port allows the user to chat with others by plugging in a headset.