I am really frustrated waiting for next installment of Symphony of Ages.  Is there going to be a next one?  It has been years!

I couldn't find anything online the last time I looked - I am waiting too!!

Wikipedia shows The Assassin King being part of a trilogy so it would be reasonable to assume she is working on the next book.

I haven't found anything definitive -BUT - I did find these porfessional published reviews which both indicate that there is more to come.

From Publishers Weekly
Starred Review. At the start of Haydon's lyrical sixth installment in her sweeping saga of musical magic and ancient prophecies (after 2004's Elegy for a Lost Star), the dragons of the world gather to mourn the loss of one of their oldest and greatest—whose demise leaves a weakness in their protective shield of the Earth. Ashe and Rhapsody, the Lord and Lady of Cymrian, also convene with their allies to prepare for the war looming between deadly powers that could tear the world apart. Rhapsody has the added distraction of caring for their infant son, Meridion, for as the heir to Cymrian the baby is an obvious target; he may also be the Child of Time, whose coming will change the world—and perhaps even the nature of Time itself. While deftly managing a large cast of intriguing characters in a story that's both grand and intimate but never predictable, Haydon moves all the pieces into place for the next volume. (Jan.)
Copyright © Reed Business Information, a division of Reed Elsevier Inc. All rights reserved. --This text refers to the Hardcover edition.

and

 Booklist
The sixth Symphony of Ages novel portends a turning point. A mysterious figure wanders about reciting, among other things, the true name of Achmed, the assassin king of Yloc--and all good fantasy readers know that names have power. Meanwhile, a conclave of dragons, few in number but vastly powerful, plots. Achmed calls a council of war, and he and his advisers agree that war is coming, on a scale that should make the trilogy that will almost certainly follow this book thoroughly gripping. Haydon isn't the greatest fantasist, but her characterization skills and her use of folkloric material have steadily improved. Roland Green

So I guess we keep waiting.

Ruth