Maybe the strings will be made of a more vibration capable material?
Seriously though, how does one play a rock harp anyway?
Sounds like a philosophical question doesn't it.
For now though, dwarves (or anybody else in DF) don't actually play the instruments, so it's kind of moot. But I see what you are getting at.
Anyways, it sounds like the new critters will have the same randomness as the new titans do. So, I guess this would mean that as a group, they are all titans, but the ones we know are titans are the ones that didn't go underground (or escaped, depending on what mythology you want to write) and the ones underground are the titans that stayed there (or were sent there, or whatever) were lost to the mists of time and pre-history.
I can just imagine a bunch of dwarves exploring a cavern when they hear a strange sound and as they enter the cavern, their torches partially illuminate a nearby ledge and as they get closer, the brightening light reveals a strange creature unlike anything in their lore, or history. The creature moves forward a bit and they get a good look at something which could only be described as a cross between a GCS, the Aliens xenomorph, and a centipede.
It lets out a horrible screech and the dwarves decide to get the hell outta there before it decided to attack.
Stuff like this will make for great storytelling.
Oh yea, Will the new Titans and their underground kin have members which are 'friendly' to dwarves and won't attack unless provoked? It would be cool to have some that aren't hostile to dwarves.