There was a recent post about this issue on the forum, although I no longer see it. Perhaps it was a victim of the recent forum upgrade.
Removing echo from a recording is not a simple task, and the results are rarely perfect. You can make an attempt at minimizing echo in Soundbooth by opening the REMOVE A SOUND task in the Tasks panel. This will open the Spectral Frequency view of your file, which may show echo in frequency ranges that are above the fundamental audio you wish to keep. You can use the Lasso tool to select the echoing audio, and reduce it's volume, or delete it altogether.
If you use Windows, you may want to download the trial version of Audition and use some of the higher end tools there. The Center Channel Extractor can be useful in isolating desired audio and removing unwanted sounds. There are other tools that can help as well, but the results will definitely depend on the quality of the source, and the amount and type of echo.
Thanks both for your inputs, I'll look at Audition.
What I don't understand is why 2 audio software within Adobe?
I just purchased through my company the "Production Studio".
There is one video software, Premiere, there is one photo editor, Photoshop, .... I don't know how to explain them that "Soundbooth" is not enough to manipulate audio files.
Soundbooth is intended to provide the kind of audio functions and working environment that people primarily working with video and flash will feel comfortable with. Audition is intended more for audio specialists (some of whom might have some involvement with video).