This is an overview of what I know about the situation with audio support in Moodle which I compiled for a faculty member who is interested in having his Comp Sci students do a project with Moodle.
I should add that our main interest is in the use of audio for foreign language teaching, both conversation practice and homework assignments. Your interests may be different.
1) Moodlespeex
Moodlespeex is a simple java client, used for recording and playing. It hooks into both the Files area and Forums. It was first developed for Moodle 1.6 by an individual developer named Dan Stowell.
It is abandonware. It hasn't been developed since about August of 2006.
I got this to work in Moodle 1.6 on files, but not forums; but both worked in Moodle 1.7. Other people have had the opposite experience. But both are clearly broken for everyone in Moodle 1.8. It does not use the standard block api, but was directly hooking into the Moodle forum code, so it is no surprise it does not upgrade well.
When it worked, it got good reviews from faculty, mainly because it is ultra simple and the quality of the speex format is very good for what foreign language teachers need.
Here is the Moodle plugin page: http://moodle.org/mod/data/view.php?d=13&rid=418
Here is the main discussion forum: http://moodle.org/mod/forum/discuss.php?d=50265
2) Covcell
Covcell is a 2 year project funded by the EU, sponsored at several universities. If you ever wanted to go to Iceland, the next Covcell conference is in Reykjavik this fall.
Covcell is actually a suite of applications mostly in Java that will run off the Red5 open source Flash server project (http://osflash.org/red5) also being done in Europe.
It is still somewhere between alpha and early beta in progress. I tested the online demo site a couple of months ago and liked it, but I could never get the streaming server installation to work. For one thing, it was completely undocumented.
Also, Covcell has a rather different priority set (chat is first on the list) than our faculty who are mostly interested in conversation practice / recording homework.
Main site: http://covcell.org/
The main audio recording software: http://covcell.org/index.php?option=com_content&view=article&id=83
Covcell Moodle discussion forum: http://moodle.org/mod/forum/view.php?id=5368
3) Gong
Gong is a fully mature Java server (Linux or Windows) based application out of the Hong Kong University of Science and Technology (http://www.ust.hk/en/index.html).
It can run as a standalone application or as a Moodle plugin. It is essentially a discussion forum application that is fully voice enabled. It has a very nice interface.
I tested this in Spring 2006 with a faculty member in Foreign Languages but she complained about the poor quality of the audio.
Back in June, I installed the new version, now up to version 5, but had the same issues with audio recording quality.
They have switched to a speex format for their chat module, but the main boards still use wav. The main problem seems to be the codec is too good and picks up a lot of extraneous noise at the high end of the frequency range, which is not needed for voice applications.
The result is a sound quality that is tinny, thin, and background noisy.
So one of the issues with audio will be the acceptability of the sound quality to your customers: the foreign language faculty. Gong would be the clear winner if only the sound quality were better.
Gong plugs into Moodle in the cleanest way: it uses the official block api and is just that: its own block. As a result, it is usually not an issue when upgrading Moodle, which is very nice.
Main Gong site: http://gong.ust.hk/index.html
Main Gong Moodle forum: http://moodle.org/mod/forum/discuss.php?d=50418
The Gong developers are very active in Moodle & very helpful.
4) DimDim
There is a piece of web conferencing software called DimDim which is being integrated with Moodle. I looked at it once last year, but it seems very different from what foreign language faculty are interested in at the moment, but it could be very useful down the road.
Main site http://www.dimdim.com/
Moodle Forum http://moodle.org/mod/forum/discuss.php?d=50760
5) MoodleVox proposal for Moodle 2.0
There is a proposal to bring something called VoiceXML to Moodle 2.0: http://docs.moodle.org/en/Development:Voice
Moodle 1.9 is about to ship, and will be the last of the 1.x series. Moodle 2.0 will be a big leap and take much longer to ship (nearly a year) than the usual 3-4 month point release.
See the development roadmap here: http://docs.moodle.org/en/Roadmap where the Voice project is listed as "Hopefully."
6) Other useful links:
The foreign language teaching forum: http://moodle.org/course/view.php?id=31
and especially see the Voice Application forum at http://moodle.org/mod/forum/view.php?id=1868