Hmm, that's interesting. (The issue is similar to that of LIE vs LAY, which can involve a usage of intransitive LAY--where the current "preferred" usage is transitive LAY.)
The short answer: What the other posters had already said, and what you also had thought.
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And now, to confuse you ...
Usually, RAISE is used as a transitive (needing an object) and RISE is used as an intransitive (no object, but often will involve using an adjunct):
e.g. I raised my arms over my head. (transitive RAISE)
e.g. The bird rose from its branch. (intransitive RISE)
But there are found some usages of intransitive RAISE (lacking an object).
If you are writing a paper or something for a formal or somewhat formal register, then you'd probably want to stick to the "preferred" usages as given in general dictionaries. ... But if you're writing fiction, or even reading it, then you might be interested in that intransitive RAISE, and so, perhaps consult a usage dictionary so you'll end up feeling comfortable.
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As to Microsoft Word: I'm guessing their software got confused, and it thought that your example was providing an object--"
an octave"--to that main verb "
rose", and so, it complained since it considered "rose" to be intransitive. For an experiment, temporarily insert the word "
up" in there and see if that makes Word happy:
My voice rose up an octave and it became increasingly difficult to think straight.
maybe that'll help Word out, maybe it won't.
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In a dictionary is:
Their example seems to be familiar.
