It would be Ok as long as you fast-forward the opening credits.
I'm curious to know why you think ENT would hold better than the others? If I were to pick a series that I can imagine holding well on audio, ENT would hardly be the first to come to mind.
Generally, Trek series could hold on audio much better than the movies, which are far more action-oriented and visual. I'm guessing that TNG might hold well on audio since it was quite "talky". DS9 would hold very well except for the episodes that had a lot of action or space battle scenes; but there are quite a few episodes like "Duet" that are based on dialogue and character interaction, that I can imagine working very well on audio. You would lose the advantage of seeing the emotions on character's faces, but most of them acted very well with their voices, too.
The ideal ENT opening credits would be a silent movie -- visual with a mute button!
Just based on my own impressions of the shows, I've always thought ENT was both dialogue-heavy and visually uninteresting. I'm generalizing, of course, and no doubt there were exceptions somewhere in the four years.
Funny you mention DS9's "Duet", I was thinking the same thing, that it would be a fine audio drama. I contrast it with "Emissary", where all the Wormhole Prophets scenes were heavily dependent on visuals and couldn't be made to work as audio.
True - the episodes where Prophets feature heavily would lose the most without the visuals. On the other hand, DS9 episodes without a lot of action or SciFi/mystical moments could work well as audio-dramas - "Duet" being a great example... and if you played them to a non-SciFi fan who has is prejudiced towards Science Fiction, they may not even realize it was Science Fiction, without seeing the actors' alien makeup.
It's true that ENT was visually lacking for the first 2 seasons. Season 3 Xindi arc in ENT has the ship in the Expanse where we get to see it caught in a lot of anomalies - and it's not like in VOY, "oh look there's an anomaly" and the ship goes into a time warp or something, but you really get to see
some weird things happening inside and outside the ship. There's also more space action, and space scenes in general - such as a look at the Spheres themselves. It helped that the show seems to have gotten better production values between season 2 and season 3 - and it gets a lot more interesting and impressive than in the first 2 seasons.
Besides, Andorians would just not be the same without their moving antennae.
I guess one thing that might help ENT is that people would probably take T'Pol more seriously if they didn't see her catsuit... But I think that there would be a problem with her and every Vulcan in a substantial role in Trek, since without the picture you would be less likely to discern their emotions. Sure, Vulcan usually don't show emotions as obviously on their faces as most other races, but most of them show them more subtly in their facial expressions, more so than in their voices, or at least I think so. I wonder how Spock-heavy TOS episodes come across without his characteristic eyebrow raise?
Which leads me to the question, which characters would work well on audio only, and which would not? I think Trek in general would do quite good as audio-only, since many actors were stage-trained and had resonant and expressive voices. Also, how would the perceptions of each character change if the audience didn't know what they looked like? I'm going to take a guess that Seven of Nine would be perceived very differently, and that most people wouldn't even realize that she was supposed to be attractive, until another character mentioned it (Kim, EMH...), they'd just hear a robotic, socially awkward and almost asexual ex-Borg.