Here's my experience with the Carillon.
Immediately after I purchased it, I targeted the bard shopkeeper, sung
the song, and then I checked her prices. I noticed a decrease in prices,
by about 1g or so for some of the higher lvl songs she sells.
That was it, I've never been able to get it to work again.
The guildmaster won't talk to me about the quest (since I'm human i
suppose) so I tried using Carillon on her 10 times or so, and she still
refused to talk to me.
So with my experience, the song DID in fact work. But once, and only
once.

As for Libretto, aye, its quite worthless indeed. We need to bug the GMs
about these two songs. Please fill out some /feedback forms or email
them or whatever. Petition if you want to. Otherwise these
bugged/worthless songs will remain just as that: bugged/worthless.

Maev
Tarew Marr