In the Inheritance Cycle, are there any instances where the Eldunari are mentioned, before Oromis explains it to Eragon? I mean, apart from the point in Brisingr where Eragon talks to a wounded solider who says he saw multiple souls within Murtagh.
Not directly, but Solembum tells Eragon about the Vault of Souls inside the Rock of Kuthian. I think I remember Glaedr and Oromis hinting at it, but they never explained it directly to him anytime before their talk about Eldunari. Oromis says he thinks he knows where Galbatorix's power comes from - the Eldunari, of course.
There is an instance in Eldest right before Eragon and Sahpira leave Ellesméra to fight in the Battle of the burning plains. It is Glaedr's last line in the book.
Extending his neck, he [Glaedr] touched the tip of his nose to Saphira's, hit gold eyes glittering like swirling pools of embers. Remember to keep your heart safe, Saphira. She hummed in response.
Keeping her "heart" safe is obviously a reference to the heart-of-hearts or Eldunari.
The Eldunari were mentioned earlier but in an indirect way. It was an instance when Eragon was fighting his second battle with Murtagh, and Eragon felt a great power within Murtagh which was not his own, but was being supplied from an unknown source which were actually the Eldunari as was eventually revealed later in the series.