Sound definately needs a comms, as well as their RF tech backstage. I've been through plenty of situations, both professional and educational where a mic has either gone done or was sweated out and had to be replaced. I need to communicate that to my wireless
wrangler, and radios make too much noise.
I've never done a show yet where it was imperative that the
conductor/music director needed to be on comms. If we're short on beltpacks (our weakness), I make sure at least one person in every position has one. The Muny and most touring shows have a comms
handset at the
conductor's
podium, but it's not used too often.
Cue lights, as mentioned, are a better way to
cue the top of the show or the top of act II.
If people have to talk over one another on comms constantly, it's time for a multichannel
system; one for SM calls/emergencies, one for everything else. The Muny uses 4 channels: one for SM calls, one for electrics, one for sound, and I think the 4th is for the
deck crew. All supplemented by Motorola radios.