Priot to this post I began contact with an agent from HP online support.
I mentioned my thoughts about the soundcard initially, and was fobbed off with "This involves equipment which falls outside the HP warranty", which is fair enough, but the further testing I've done practically elminates the soundcard. Currently the agent is "doing further research into the matter" and promises to get back to me
I have plenty of spare (with suppressor thingies) USB mini-B cables lying around, and a couple of spare non-suppressed large-size device cable, so I'll try splicing a couple together tomorrow. Failing this, I can't return the printer as the person who bought it doesn't really see my speaker noise as that important
and thus doesn't want to return the printer - so I may invest in a cheap network print server (Netgear PS121 on ebay for £26.95 looks ok).
The cable was supplied with the printer, but I've tried other cables to no avail.