Two episodes of The Invasion were animated and set to the soundtrack. No other episodes have been animated, and it cost so much to do those two that it seems unlikely more will follow.
There's some kind of "reconstruction" thingie around, whereby some techno-whizzo people have set the soundtracks to still photos, but I don't know if those exist for every missing episode. I haven't seen any of them.
A couple of stories (Ice Warriors and Tenth Planet, if memory serves) are iout on video with their missing episodes filled in with bits of soundtrack and a few photos. Doesn't give the full effect though, as the missing episodes are reduced to five or ten minutes each.
There are 110 missing episodes of DW (I think. It might be 109. Or 107). The soundtracks have been cleaned up nicely, and filled in with suitable narration for the visual bits. I am completely blown away by them. I never thought I would ever know anything about, for example, The Myth Makers or Fury From The Deep, but they are both magnificent works. Marco Polo is beyond epic. All tragically overlooked because they aren't sat on the DVD shelf and require a bit of effort to sit down and listen to.
The "odd episodes" (Faceless Ones, Crusade, etc) that still exist seem orphaned from the stiory and fairly meaningless on their own. I have used the soundtrack to fill in the blanks, and the whole story pulls together very neatly. The Moonbase really shines when you're able to experience the whole story.
I am re-evaluating everything I have ever thought about the earliest years of DW because I am now experiencing huge chunks of it for the first time. It's a real ear-opener.
All those people over in the "Classic Series Companion Survivor" thread are voting for Katarina as least favourite companion just because they know nothing about her and it's easy to get rid of her. Her death scene, and the Doctor's eulogy afterwards, and precisely fifty-seven times more moving than all those snot-and-tears melodramatic "I love you Doctor" outbursts that seem to follow David Tennant everywhere.
These soundtracks just go to show: in Doctor Who, as in the rest of TV, dumbing down is rife.
Lined up and waiting to be listened to: Power of the Daleks, Galaxy Four (I have always wanted to know more about that story! Read the Target book recently, and it just got me more intrigued!), The Macra Terror and The Enemy of the World. All of them so chock-full of mystery and unknown content that I really don't known where to start!
Go on ... treat yourself. Get The Web of Fear or Fury from the Deep. Don't wait for them to be rediscovered, or animated, or reconstructed, cos you might have a blummin long wait.