Hi,
There are several theories about the sound and the metal of the case but AFAIK pink gold has often been recognized as the best sounding precious metal.
Titanium and steel have been also recognized as sounding very well, if not better than pink gold (AP or JLC for instance have experienced these materials, maybe GP too).
Furthermore FP Journe for instance has chosen steel as the best metal for its repeating watches (I've never been able to hear any by the way).
Then there are other aspects.
First the gongs: the cathedral gongs of the Patek Philippe 5074P (heard last summer) have given the best ever (all brands included) to my hears, even better than what my memory remembers of a pink gold 5016 heard long ago (more than ten years ago) or an extraordinary pink gold skeletton tear drop VC owned by Claude-Daniel Proellochs himself (although the three tear drop VC I heard were all excellent, one in pink gold, two in platinum) or a Grande Sonnerie wristwatch from Philippe Dufour.
The sound of all "simple" gong platinum PP watches I heard was excellent but not to the level of cathedral gong models.
That's the reason why I would choose a 5074 if I had the hard task to choose a PP wrist minute repeater. I'd love to hear a 5074R and compare with a P to hear if it's even better!
I love the 3939 for its simple pure and perfect design, or the 5016 for its blend of functions, but the sounds rules for me.
The 5013 is another underappreciated watch for its original and rare (for PP at least) look that I appreciated a lot when I saw it.
Another aspect: the question of which metal is based on the principle that the sound comes through the case. That's certainly true but probably partly only.
Jaeger-LeCoultre has shown in the last years that the sound also comes through the glass and has consequently issued fantastic watches like the Master Minute Repeater which titanium version is particularly excellently sounding (still not comparable to the PP cathedral gong IMO). The new Master Grande Complication issued last SIHH or the Duometre a Grande Sonnerie will be certainly interesting to hear once production pieces will be accessible.
I would advise to do some search on our past subjects (not just PP) about the sounding of repeaters. Several have posted (among them our own Dr Thomas Mao) very interesting articles on that subject.
To conclude I would add one personal aspect: if I really had money to buy such pieces, I would look for a top pocket watch, or would commission a pocket watch from PP with cathedral gongs! That would be something!
Cheers
Dje