I'm not aware of any of these being issues with the demo license.
Upload MP3 Files -- http://communityserver.org/forums/t/488521.aspx
New members can't sign up -- Are you using a Single Sign-on Module? Are you getting an error?
Mirrored blogs full text -- Is the source feed providing the full text?
Comments on mirrored blogs -- Most themes disable support for comments for mirrored blogs. You can remove the
<DisplayConditions Operator="Not"><CSBlog:WeblogPostPropertyValueComparison runat="server" ComparisonProperty="IsExternal" Operator="IsSetOrTrue" /></DisplayConditions>
from the blog theme's post.aspx page. This will allow comments to be entered/shown for mirrored blog posts.
Need to re-login every 5-10 minutes -- http://communityserver.org/forums/p/486642/577217.aspx#577217