The <mumble>mumbling</mumble> notation used for Ophelia's muttering in panel 1 is based on HTML markup, which no doubt many readers recognise instantly. It's become a de facto standard for indicating the context of words or phrases in otherwise plain English sentences amongst those of a geeky persuasion, since it has the ability to indicate any mood by explicitly spelling it out, while at the same time indicating clearly that the mood is not part of the words actually being spoken, but merely a marker to indicate the external context.

As such, it's a clear and obvious way to show such things in written dialogue, and I couldn't come up with any better method of indicating that Ophelia is muttering under her breath to herself. I know comic artists have invented various graphical devices to indicate such things in the past, which are easier if you're hand-lettering your dialogue, but this method works and is geeky into the bargain, so makes sense to me.