I had the same problem with audio (probably with video too, but since I don't have a webcam I can't tell).
If you have a speedtouch 510 this may help. The modem uses the port 5060 for the SIP protocol or something. So you have to unbound this.
Open terminal and type:
telnet 10.0.0.138
This will show up:
Trying 10.0.0.138...
Connected to speedtouch
Escape character is '^]'.
Username :
put your usrname
Password : ********
your pass
And then you will receive the welcome screen.
Now, to unbind the SIP protocol type this:
=>nat unbind application=SIP port=5060
=>config save
And then, restart your modem
=>system reboot
There you go. I can audio chat perfectly since then.