The original stagecraft newsgroup. This was supposed to be replaced by rec.arts.theatre.stagecraft but like many alt. The 'official' stagecraft newsgroup. Lots of noise, but lots of signal too.
Discussions are mostly about set, props, lighting and sound but any backstage topics are welcome.
The Usenet2 stagecraft newsgroup. A lot of good stuff here. Most people are discussing studio recording rather than sound reinforcement, but there's a lot of overlap. Just one word. Look at rec.pyrotechnics instead.