First I am not a qualified electrician, but I was trained by MoD as an electronic engineer.
Loud speaker cables are low voltage sos not that dangerous. BUT you are in a high moisture content room. Moisture conducts electricity!! And thus a speaker volume control that is not rated corrrectly could cause DEATH by electrocution!!!!
When I say rated I mean against moisture. There is a standard for this, its the IP (ingress protection) scale. You can find details here:
http://www.youpoc.net/ingress-protection-rating
I would suggest using a device that has an IP44 or above rating.
I dont know if a speaker volume controller with an IP44 rating exists but that would be the safest option.
I do realise that all the above seems very much overkill but you wouldnt half be sorry if your speaker controller killed a loved one!!!
As for building regs for speakers in a bathroom I dont know, but I do know that ANY mains voltage work in a bathroom or kitchen for that matter a suitably qualified person is required.