After updating Capybara, all my steps featuring the audio element started failing. After asking around and googling, I came across this explanation for how Capybara handles hidden elements.

Capybara now has the property ignore_hidden_elements default to true. As an audio element is considered a hidden element, it is no longer visible to selectors by default.

You can change this default behaviour by adding Capybara.ignore_hidden_elements = false to your support/env.rb file, or you can add :visible => false to individual queries.

All that was required to get my features passing again; was changing my step definitions, from this:

page.should have_selector('audio#elRaptorShriek')

to this: