Icecast - Jun 2012 - Jack trouble- cannot lock etc/passwd (Icecast/IDJC)

I`m making my radio station with Icecast. On Windows desktop everything
worked perfectly but on my private Ubuntu-based laptop it`s not so easy...
I` m using a tutorial here: 
http://idjc.sourceforge.net/tutorials_icecast.html
When I try to run Jack server I get something like this: "Jack is running
in
realtime mode, but you`re not allowed to use realtime scheduling. Your
system has an audio group but you`re not a member of it. Please add yourself
to the group by executing as a root: usermod -a -G audio (my username)"

I wrote this line in the terminal but all I have now is:
usermod: cannot lock /etc/passwd try again later.

What it is? I`m afraid it may be something from the previous experiments
with Icecast- I had uninstalled everything that could be unnecessary but
something could stay. I`m an absolute greenhorn in Linux and I feel really
unsure in the terminal. Guys, if you have any ideas what`s going on please
write me fast. The official premiere of the radio station is planned on
Friday and it had been already delayed once - I don`t want to do it again.
:(
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On Wed, Jun 13, 2012 at 03:57:14AM -0700, Mavoy wrote:
> realtime mode, but you`re not allowed to use realtime scheduling. Your
> system has an audio group but you`re not a member of it. Please add
yourself
> to the group by executing as a root: usermod -a -G audio (my
username)"
> 
> I wrote this line in the terminal but all I have now is:
> usermod: cannot lock /etc/passwd try again later.
Try:

  sudo adduser $USER audio

Then log out completely and log in again.

-- 
Paul Martin <pm at nowster.me.uk>
also <pm at debian.org>

Paul Martin-10 wrote:
> 
> 
> Try:
> 
>   sudo adduser $USER audio
> 
> Then log out completely and log in again.
> 
> -- 
> Paul Martin <pm at nowster.me.uk>
> also <pm at debian.org>
> _______________________________________________
> Icecast mailing list
> Icecast at xiph.org
> http://lists.xiph.org/mailman/listinfo/icecast
> 
> 

"audio group already exists".
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