Bibtex bold / boldface in journal article volume

Declare a function "embolden" if it is not already defined in your .bst

FUNCTION {embolden}
{ duplicate$ empty$
    { pop$ "" }
    { "{\bf " swap$ * "}" * }
  if$
}

Use it in the following pres-existing passage of your .bst:

FUNCTION {format.vol.num.pages}
{ volume embolden field.or.null

Note, the order of declaration is important, so declare the first function upstream of the function where it is used.

Inspiration: aip.bst from http://www.ctan.org/tex-archive/biblio/bibtex/contrib/phy-bstyles