I would say have a lot of keyboard shortcuts and sounds to go with the text. Ambiant sounds help with environments as well...
$1I would also post in the developers room of the audio games forum. Developers would have interesting ideas!
Not sure if spoken sound files can be included in the MUD but that would be very unique and never done before.
But for me... I have a lot of trouble building a mental map of places im Muds. I get lost very easily. Not to mention... in more cases than not, I have ran into situations where I constantly need to keep a notepad open of spellings and commands so I can just copy and paste. If these could be shorened or bound to a macro or something that would be good.
Also, fast travel. Make Fast travel a thing.