Thanks for the updated icons uxdavid. As you have no doubt noticed, I have put them into the forum for a trial period so that we can gather feedback from forum users.
There has already been some "off-list" comments from some of the moderators and helpers on this forum, which I will attempt to summarise. Note that these views do not necessarily represent the views of all of the forum crew, just the feedback that I've received so far. It would be nice if we also got some feedback from users of the forum (invitation for forum users to comment
There is overall agreement that the choice of images depict the topic that they are intended for, though there is concern that they don't fit well with the overall look of the forum.
In particular, the chosen shade of blue looks garish against the pink (example near top of page), though that may be more about the use of pink. On the index page
the blue is noticeably less green than other blues on the page, which looks a bit odd in context. Although I note that your choice of blue was based on the use of blue in the Audacity logo, the relevance is not immediately apparent as it represents just one of the three strong colours (and is a lighter shade of blue).
I think that you have successfully maintained a consistent style across the range of icons, though there have been comments that they look a bit "blocky". To quote one comment: "They might work on a modern looking site with large blocky elements
There was also a practical consideration raised by one of the moderators, which was that with just one colour, the individual icons are less distinctive, so it is less easy to recognise at a glance the specific forum board that you are looking for.
In summary, the icons themselves are clear and have a consistent style, (both are qualities that we want), but that the monochrome style does not fit well with the overall look of the forum. One solution would be to redesign the entire forum, but I'd rather not go down that road at the present time due to other priorities (such as helping to get Audacity 2.1.2 ready for release).
I hope that this feedback is constructive, and invite others to comment while the icons are still up on the forum.