I cannot really talk about audiobooks, since I only heard one of these things in my life, but it seems they can be interesting, and people who don't have time for reading, or cannot read, partially sighted persons for instance, can discover new books through them.

About adaptations... Well, some great works touched an even greater audience through their adaptations, like Lord of the Rings or A Song of Ice and Fire, but once you read the source material, you can't help but feel that the adaptations will often butcher the source material. Let's not talk about the translations as well, in one of the french translations of the Dresden Files, someone thought that "vanilla mortal" meant "mortel à la vanille", or the fact that you can end up with two books for one english book after the translation ...

I have no real opinions about the other questions, except I prefer, because there is potential for a longer story.

I feel i cannot yet give any sort of reasonable opinion about audio books
because I have exposed myself to the medium precisely once. All I can say is that I like that particular audio book and that I am not disinclined to try another one.

I think that it is pretty much inevitable that something will be lost in translation, so to speak, when it comes to adaptations, especially when it is case of any longer medium being adapted to film and even especially when comes to any sort of interactive media being adapted to a non-interactive one.

That doesn't mean that they have to be necessarily bad though, just severely diferrent experiences. I can even account for at least one case where the movie adaptation was a major improviment on the book.

With the choice of media, I think it mostly comes down to how you want to convey that message. Books and Visual Novels generally allow you to peer more closely and deeper into the psyche of at least one of characters involved because of the necessary presence of narration. That doesn't mean that you can't relate meaningfully to characters in other media, which the contrary, just that relationship will present itself under a different form. Every medium posesses traits that makes its experience radically differently from any other and without which the experience probably wouldn't be able to work in the same way.

Take Visual Novels for example. One of my favorites stories out there is Kana Little Sister. Would that experience have worked just as well without any sort of interactivity, without all the different choices, routes
and endings? Probably not. But would that experience also have worked without the intimate relationship created by the narration provided by protagonist, without the extensive amount of text involved, the CGI backgrounds, or even the wonderful background music? I am also willingly to wager that it probably wouldn't have.

I love audiobooks because they're super easy to multitask with- drawing, commuting, whatever- though a definite drawback is time. It's generally a little faster to read the book yourself than listen to an audiobook. For me, audiobooks are generally more relaxing, especially if they have different voices for different characters. :P I don't believe intrinsic flaws exist in either medium; both have pros and cons.

Don't have any particular opinions in adaptation! It really does vary from situation to situation. Good adaptations probably ultimately add to the interest of the work as a whole. Bad adaptations do the opposite. Of course, this then breaks into the semantics of what constitutes "good" and what constitutes "bad", which can obviously vary from person to person, so.

As for book vs. graphic novel, some stories are just meant to be in one or the other. If you've ever read Neil Gaiman's "Sandman" comics (and if you haven't, I highly recommend them, they are so good), you might get what I mean. Could the series have been conveyed well through text alone? Probably. Would it have been effective in its message, themes, or general appeal? In my opinion, probably not!
Books with deliberately ambiguous characters or settings also benefit more from written rather than illustrated, as it leaves a certain amount of interpretation up to the reader.

And movies vs. television series? Probably more "there is way too much important footage to fit this into a single or even two movies, guess we're making a miniseries" :P miniseries can probably fit in more suspense and cliffhangers, and also have the obvious added bonus of nigh unlimited time for plot lines.