Invision doesn't use any DLL or hack to render graphical telnet sessions. It really isn't necessary. It instead uses picture windows and displays the graphics the same way that a telnet client would - moving the cursor around the screen to create the graphics instead of trying to display a full line at once. Speed is basically equal to a real telnet client unless the graphics get really crazy, which isn't normally the case. Sound isn't included simply because mIRC doesn't have a way to play a specific note for a specific length of time and trying to do it with WAV files would be a pain. The point is that it is definitely possible already. That feature has been part of Invision for about 1.5 years now. As I mentioned before, there are some very minor issues that can be seen in specific games or other places, but if someone wanted to really learn how to correctly view and act upon the telnet codes, those could all be cleaned up. When I made it, I wanted something that worked well, but I wasn't going to try to figure out everything when the interest in a telnet client is so low. Even with Invision, few people actually make use of the client and non-graphical telnet has been part of Invision for a very long time. It may be true that IRC users are more likely to be willing to play a text-based game like a MUD, but that doesn't mean there are many who will. From experience with Invision's users, telnet just isn't really something 99% have any interest in. I just can't see any reason for mIRC to add something that interests so few people, especially when it isn't really that hard to script one.
As far as telnet clients go, I've seen good clients that render everything really well. Telnet hasn't exactly changed much in the last 20 years, so any client that worked well then should work well now as long as it's compatible with the current OS. If there is something specific that is missing unrelated to displaying the MUD, you'd need to be more specific. There are a variety of clients specifically designed for certain games that offer extra features that track information about the game or display popup windows for specific events in the game or whatever else, but those aren't really part of telnet. Those features are just extras and are usually only useful for specific games.
