Thanks for the idea Bert, do you have any evidence that urethane foam would do the job? The site I linked to "sounproofing info" is pretty clear that only a high mass barrier like lead or Mass loaded vinyl will reduce noise transmission. I found that site to have a lot of good info, FAQs, and popular myths busted.
I wonder how well it would work if you took one of the standard "Hoekers" that are used for the pavillions in most festivals, and lined it with either the lead stuff( very expensive) or the high mass vinyl. Entrances would need to be baffled e.g a zig zag path lined with sound absorbent material.
I am inclined to think that sound transmitted through the ground would not be very significant. In theory a high enough wall (eg motorway sound barriers) will not allow much sound to be bounced over the top. From my own experience I think a sound proof roof would also be required...amd that is a bit of an engineering challenge.
By the way, the Singing Shed at the Woodford festival, although not as sound proof as it could have been due to no doors and not very high walls, did prove to be a very popular venue. That's why I think the idea is worth investigating.