asterisk users - Jun 2006 - One problem (MOH) and one question (incoming SIP calls)

OK, here's another problem I've run into with Asterisk.  In the
musiconhold.conf
file, I can set the music on hold mode to files and the directory to the place
where I have my MP3s stored and they play.  If I set the music on hold mode
to any other setting, instead of getting my MP3s, I get something that sounds
like a motorcycle being cranked up and driving off.  It sounds really weird,
at any rate.  I've even gone so far as to remove all the mp3 files from the
/var/lib/asterisk/mohmp3 directory and this "music" is still the only
thing
that will play with any other setting.  Is this normal or is there something
in my Linux setup (I'm running Knoppix installed to a HD) which I can check
for this?

The question: if my Asterisk server is on a domain which is accessible
from the public internet, is it possible for someone with a SIP client which
is not on my Asterisk box to be able to dial something like (myexten@mydomain)
and get an extension on my box?  If so, what would I need to do to enable
it or does it come that way "out of the box", as it were?

Undrhil