The machines are also fairly ageing, and don't support the composited desktop. I did test Ubuntu 11.10 beta on one, but it was so slow (with what I assume was Unity 2D) that it would be unusable for day-to-day work. All we use at work are Chrome, Thunderbird, OpenOffice and Audacity - why would we need Unity? Why can't we turn it off?
For me, Ubuntu definitely peaked at 10.04 and I'm going to be looking elsewhere in the near future.
Now, Sir, Just what do you call 'older'?
Our older users are in their 60s and 70s. We're a non profit radio station and several of our volunteers have never used a computer before, or have had only a passing involvement with them.
What sound software do you use? Does it involve Jack?
Looks like you may be able to keep Ubuntu with a current release and configure Gnome Fallback, looking like Gnome 2 to your users.
(Apologies for multiposting - just saw this)
On the other hand, if you're looking for something not too far from Ubuntu, Debian or Fedora might be better options.