Problems with soundconverter

[Wilfred Zegwaard <wilfred.zegwaard@gmail.com>]

Hello,

I'm a novice debian linux user and I can't get soundconverter to work. I
want it to convert a number of flac files into mp3 files. What I need is
gstreamer0.10-lame. It is to be found on the debian multimedia site, but
when I add the debian multimedia repository to sources.list, it returns
an error:

W: GPG error: http://www.debian-multimedia.org etch Release: De volgende
ondertekeningen konden niet geverifiëerd worden omdat de publieke
sleutel niet beschikbaar is: NO_PUBKEY 07DC563D1F41B907

What am I doing wrong?

Wilfred