So I'm looking to encourage MP&T users to commit to checking out the room(s) monthly, weekly, or daily, respond to those topics that interest you, post conversation topics when so inclined, and generally encourage use of the chatrooms by suggesting them to posters and commenters on the main site and Meta.
Another suggestion I would like to add to the above is to make use of chat events (these can be one-time or recurring). Upcoming chat events are listed in a banner at the top of the list of all chat rooms, and curious users sometimes drop in to see what's going on. Previous experience with such events on a chat room linked to Music Fans suggests that more than a few users tend to stick around even after the event is done.
I believe moderators also have a tool to schedule events, which then appear in the community bulletin. I know that this is put to full use on Literature, so one can take a look at how they do things to get an idea about how useful this tool could be on this site.
Here are some suggestions for chat events, as requested in the comments:
One simple idea is to have a recurring event, perhaps weekly, on a casual theme. A "hangout event for X" where X could be anything — Bach enthusiasts, cello enthusiasts, waltz music, etc.
Since I haven't participated in The Practice Room, I can't really say anything more specific. But the idea is to choose a theme that is sufficiently broad that folks are tempted to drop in to see what's going on, and also allow them to chime in. It would help to choose a theme that the chat room regulars (or whoever chooses to host the event) are comfortable with, so that they can make easy conversation.
Since I also haven't participated in The Auditorium, I may be off the mark with my next suggestion. If the idea is to encourage more people to engage in the chat room by posting their recordings and start discussions around them, then it could help to have a recurring event (say, weekly) with a straightforward theme (e.g., "Weekly jam at The Auditorium. Feel free to share your tunes!" or any friendly open invitation of that kind). For one, it will reassure potential participants that they are not posting into the void if they know there's a chat event going on. For another, it lets people know that there is even such a venue on SE chat, and a recurring event will make it more likely that people won't forget that it exists.
Again, since I haven't really participated in this chatroom, I may be wrong about what the purpose of the chatroom actually is, so feel free to correct me.
Another idea, to follow the lead of Literature, is to have month-long events, that are also promoted on the main site's community bulletin, based on any relevant theme. Suggestions can be collected in a Meta thread and voted on, and so on and so forth.
For instance, if a classical composer was born in a particular month, then their works could be chosen as the theme for that month. The main chat room can host discussions on that composer's works. This could easily lead to new, good-quality questions being asked on the main site regarding that composer's works. A highlight reel can be promoted in a Meta thread at the conclusion of the event.
Alternatively, current events can dictate which theme to pick.
Another choice would be to look at under-represented tags on the main site and pick one of them in order to increase the number of views on the already existing questions and answers in that tag, as well as to promote new questions to be asked and answered.
At the end of the day, chat events make chat more lively, and this then promotes a sense of community.
Naturally, successfully promoting chat will require some dedication from the site's (and especially the chat's) users. As I see it, that is what the call to action in this thread is about. The details can be certainly refined through further discussion.
(Oh, and since I'm a room owner for Saptak, I'll add that I have some plans (tentative, at the moment) to run chat events based on specific ragams, composers, and artists. The choice will have to depend on whatever I'm listening to and learning from at that time, at least initially. But this also means that it can be quite flexible, which I feel is a good thing.)
