Isengard no!

[SoundingShores]

Perhaps Isengard is meant to be the Modern English translation of a Westron name. It was originally a Gondorian fortress. I guess at first, it had a Sindarin name, which was translated into Westron. And Westron is translated into Modern English, sometimes by using actual Modern English words (Cotton), sometimes by starting with an Old English word and making it sound more "modern" (sāmwīs to Samwise).

This may explain why Tolkien pronounced Isengard the way he did. The Appendix E pronunciation rules would have only applied to the Sindarin and maybe to the actual Westron name of the fortress, whatever it was.

Isengard may have had a name in the Rohanese language as well since it was close to them, but I think it wasn't part of the territory Cirion gave to Eorl. Rohanese is a bit complicated because it's supposed to be translated into Old English, but sometimes, the words are modernized anyway. When they aren't modernized, maybe we're meant to use Old English pronunciation, rather than Appendix E pronunciation. EDIT: But to be clear I think Isengard is the Modern English translation of a Westron word, the "take an Old English word and make it sound modern" variety.

Originally Posted by Huinesoron

... of course, as the footnote to the Vowels section makes clear, if you read the everything with English pronunciation, you "will err little more than Bilbo, Meriadoc, or Peregrin". So Tolkien was reading his poem with a Shire accent!

This reminds me of that funny little anecdote in Appendix F about Pippin naively and inappropriately using the familiar "you" (and he/she/they, apparently) when he was in Minas Tirith because the formal versions weren't used in the Shire.