There is a loudspeaker manufacturer who did trial brass screws, and used them for some time, until he could find someone who could manufacture him some copper ones.
Now, I do not know the purity of the copper screws, nor the exact composition of the brass. The brass screws did make a difference to the sound, and that difference was sufficient that the less expensive steel screws were dropped and the more expensive brass were introduced into the builds.
[I have myself had copper machined and cut for projects, waterjet and cnc routed. I acknowledge that small amounts of another metal turning the copper into an alloy, makes it more readily machinable]
The correct torque settings were discovered, and I haven't confirmed how that was arrived at, however I do believe it was many hours of R&D.
At the factory, a torque setter is used, and the screws have an Allen head on them. They are also secured in place with Loctite on the threads.
The old brass screws are still being used, however they are used in the tuning bay on the back of the speakers, simply as a conductor. The screws pass through the board the crossover is built upon, into an open bay in the back of the speaker. The tuning bay is used after some run in time on the speakers, whereby placing a resistor (and sometimes a capacaitor) in parallel with resistors in the crossover circuit to lower resistance, and do a final tune using Clio measurement system to dial in the final crossover measurements to very close tolerances. Having a closely matched pair makes for a very good soundstage.
Also a newer shaped baffle made of copper (an alloy of copper) did yield an audible increase in fidelity, on speakers with decoupled exotic tweeters.
This was used in conjunction with a 2" thick acoustic foam covered baffle.
