Does a "one key" voice recording application exist? I am looking for a voice recording application that could be used in the same way as a dictaphone, by pressing one key (for example "volume up" during a few seconds) without having to unlock the screen and find and click the shortcut to the application. Such an application could easily be used while driving.

Does such an application exist ? Is it even possible for applications to use key triggers while the screen is locked ?
Something like that would have to be done in a custom rom, there's no way in the api to do that.  I'd suggest looking at something like widget locker to put a slider on your lock screen that takes you straight to your voice recorder app.