Hi Pjago,
Curiously have you tried to reboot or restart the iPhone as well as the SoundDock XT? Have you confirmed that the volume is up by pressing the + button multiple times to the right of the docked iPhone? Also, have you tried to press the + and - buttons on the sides of the docked iPhone for two seconds, then release? This should switch the input to the docked speaker. Do you have a case on the iPhone? If so, have you tried to remove the case and re-dock the iPhone? Lastly, have you tried a different iOS device docked in the speaker? This would help tell us whether the issue is with the dock or the iPhone.
If the above does not resolve the issue, I would recommend contacting your local support center. If you go to our global page HERE, you can select your country, then scroll down to the bottom of the page and click "Contact Us". This should provide you with the phone number, email, and chat services for your country.
Thank you,
-Michael