Ossë that's a great interpretation! Perhaps it was not
Pippin's stone, or the drums of the
Orcs, or anything else that drew the Valarauko to the Fellowship (after all, he was leading the nasties, not the other way round), but
the Ring around
Frodo's neck! I haven't even considered that actually, as I'm sure the Fellowship wouldn't have in such a distressing situation.
Well, seeing as the physicality of the
Balrog has quite a direct relevance to this topic, I'll share my thoughts on it after all.

I hold that, as essentially Maian beings, the
Balrogs' form was neither wholly metaphysical nor wholly physical; i.e. they existed neither solely in the Seen or in the Unseen. Also, as powerful
Maiar, they would (like other characters in
Tolkien's works) have been able to shift between Seen and Unseen, or exist 'more' in either of them.
I hold also that when the Balrog was confronted by
Gandalf, and when he 'stepped' onto the Bridge, he existed 'more' in the Seen world, and thus was able to interact with his surroundings. There are many possible motives for his doing so, perhaps to try and frighten his adversary away or to be able to battle him better. Also, when the Bridge collapsed, the Balrog was taken by surprise and upon being attacked mid-drop by Gandalf, he would have been unable to shift back to the Unseen. But I would say that after the fall in the water, and the wrestling match between him and Gandalf, the Balrog was able to regain his more Unseen form, which is why he 'flew' up the stairs and into the maze of tunnels under
Moria rather than labouring up them clumsily, as his size would have undoubtedly made for.
Then, again, on the mountain-top both the Balrog and Gandalf (in my view) were more in the Unseen; how else would either have been able to really hurt the other? And lastly, I hold that when Gandalf managed somehow to perhaps tap into the Balrog's
hröa-driving force, his life-source you might say (though not the
fëa), and was able to defeat him and, in essense, 'kill' him, the Balrog became entirely existant in the Seen world and thus 'smote the mountainside' as a heavy, large, physical mass.
There are probably flaws in this reasoning, but it's something I've been tossing around for a little while.. And given the Balrog's hypothetical skill of Shifting between Seen and Unseen, it would tie in well with the Ring's own Seen/Unseen-link and could provide the Balrog with some means of utilizing it.
But I have a theory about the Ring as well: I think that
Sauron did not only design it solely for himself, but also for the specific humanoid form he was in. Had he taken the form of a dark cloud or something else, a material object like the Ring could presumably not be used while in that state. The purpose of the Ring was only to increase Sauron's magical capabilities
while in humanoid form, which made his 'world domination'-plan far more executable. The invisibility factor is, to me, a pure side-effect; as a
Maia, Sauron already existed in part in the Unseen and so would not turn invisible, but Frodo as a wholly Seen entity would be transported 'mostly' to the Unseen upon tapping into the
evil energy in the Ring, i.e. becoming invisible.