If you were to walk into hmv or some other outlet, chances are you'd find part 2, and occasionally you might find other imported parts of the series. However, anyone who does their research on this will know that in order for a cd to be on official release, every artist featured must have provided clearance for their track to be included. 'As Heard on... Part 2' was the only album which received the thumbs up from all artists featured. If you visit the official site, it details the artists which refused to officially contribute.
Remaining parts of the series therefore remain 'promotional'- much like a city-centre dj promoting his skills by touting a mix cd to interested parties. Of course, in the latter instance, the dj hasn't contacted every artist on his mix cd and therefore wouldn't be able to release it via a record company. But that's the only difference. There are 10 'As Heard on Radio Soulwax' compilations, and a further 5 'Hang all Djs' compilations, all of which are derived from the Radio Soulwax shows. These compilations feature all the unique and individual artwork that part2 employed, and are in exactly the same vein as part2, only you might not always find them in hmv!
You do find them elsewhere though. Some eBayers have sold cheap cd-r's with pen scrawled across them to identify which one's which, despite advertising them as originals. I'd want to see some pictures before buying so you know what's being sold. But don't assume that's always going to be the case, because you can purchase this series legitimately.
There is some debate as to whether or not the 2 many djs team compiled the other releases- it's been said that 'other' djs may have put them together. On a rational note, however, why would another dj spend time, energy and resources putting together a cd for which they receive absolutely none of the credit? Cds are released via waxedsoul recordings, but are labelled as promotional so that the artists included cannot sue or claim royalties. It's that simple. Soulwax on. Soulwax off.