There's always IBM's own solution to the volume level of buckling spring: the greased buckling spring.
Then there's the non-solution solution: get a Model M/F for everybody in your office, and nobody can complain about the noise anymore.
IIRC dental-floss mod gives the same effect but is more easily reversible and you don't risk the grease dripping down.
I did the floss mod on an M2, but there's no way I call the end result 'quiet'. What it does is soften the noise a little, and almost completely eliminate the 'ping'. I found this an acceptable compromise.
Having said that, the longer you cut each piece of floss, the quieter it gets, though this compromises the key feel.
As a long time buckling spring user I'd have to agree with the others - there's nothing really comparable in terms of a quieter switch.