This is most likely a bug, do you mean your speakers are constantly producing an audio noise immediately when OpenBSD boots? or is the noise noticeable before that happens?
It could be either a hardware problem, or a driver problem... out of curiosities sake, try the following:
1) Set
outputs.master.mute to
on.
2) Use the UKC or
config(8) and try disabling
pcppi(4) or
azalia(4), either could be causing this audio problem.
3) Boot and install a recent snapshot from a local mirror..
If nothing works, considering filing a bug report.. instructions are available
here.