Thanks for asking. It's nice to see interest from someone pouring yourself into some real art.
How about you have something like "Thanks to contributors of sounds on FreeSound.org:" then specific sounds and their contributors, such as a line saying "BGvesselNoise4+5_Choral.wav by Keith W. Blackwell"? And if you have room, another line with the URL (for this, "http://www.freesound.org/people/zimbot/sounds/122962"), though that probably is more helpful in electronic text (like a description posted with a video online somewhere) or even in printed credits (fine print within a DVD booklet) than as part of the in-video credit list. The same line or two-line format then recurring for each sound used from FreeSound.org.
Unfortunately, my sound names tend to be very long, like this one, making them harder to fit comfortably, so worst case you could reduce the one or two lines for this entry to "122962 by zimbot (Keith W. Blackwell)" -- in addition to the mention of freesound.org, of course. That's enough for anyone interested to track down the specific sound used (and I'm not so sure the sound name in this case would be any more helpful than its number). Anything along those lines is fine with me, and as the visual artist you have to decide how to format all your credits within the video itself. And I suppose if you ever used multiple sounds from a single freesound user, you could list all the numbers, then "by so-and-so" as one entry. Works for me. I just ask for attribution and that you identify the specific sound[s] used, but other than that I don't much care.
If the in-video credits need to be shorter, an idea that seems kind of obvious is to put the details on a web page and have the in-video credits show only a general acknowledgement with "find details at" and provide the URL -- but the problem with that approach is that you have to always maintain that web page at that URL indefinitely, so I think that's kind of risky. Similar caveats apply for saying "see description for credits" on YouTube (etc.) since the video might not stay where you first put it (and the description might not follow). You might as well keep the credits in the video itself so they are guaranteed to always go with it no matter where it goes, IMHO.
Thanks. You will probably want to envelope down the ambient after-sound (especially the LF portion, which makes it sound like it was recorded in a small room, which it may or may not have been). In fact, I would recommend using only the first 1/5 or 1/4 of this, and fade it out to nothing from the peak to the end. Upon listing and looking more closely, I think I must have taken a slinky sound, and prefixed it with a shortened, enveloped, and reversed version, but I left the original tail in place. The sound of that tail would not be appropriate for settings outside the confines of close walls, IMHO. But YMMV. I never really expected anyone to find this useful, but freesound is full of surprises. :-)
Heh heh, the first rating is only 1 star. I suppose that's because the producer wanted this to have a bit of a comical effect (in the radio play, the "broken portal" and "portal mechanics" were a bit of comic relief). As a result, I put a squeaky-toy noise and some lip pops into the composition. I suppose as a result this one is not suitable for a more serious application. :-)