'Pern' Question, concerning dragon telepathy.?

I've been reading Anne McCaffrey's Dragonrider's of Pern books, again... So far I've reread Dragonflight, Dragonquest, Dragonsong, and Dragonsinger.

There are a few instances in Dragonquest, Dragonsong, and possibly others where people who have not impressed a dragon have heard dragons speak. I particularly recall the Masterharper being surprised in the later part of Dragonquest (I think).. and there were a few other instances as well.

Of course, F'nor can hear Canth, and so forth, and Lessa could hear all dragons, but there was no reason for the Masterharper to hear anydragon, and the other instance was unusual as well.

Is this ever explained? I'm doing some research and while I've all intentions of reading everything, this mystery is one I'd like to figure out sooner rather then later. :)

Also, if you could mention which book answers this question, that'd be awesome, I'll move it to the top of the pile. :D

Thanks! (off to 'the White Dragon')