A feature request for 3.1 is to add the Opus audio codec as a core media type:

Publishers that rely heavily on Media Overlays to provide synchronized text-audio publications would, in order to achieve reduced file sizes and increased audio quality, benefit from a speech-optimized format as a Core Media Type.

Opus would also represent a royalty-free option to MP3.

Discussions to date have been favourable, as Opus is supported, or indicated to be support, in all major browsers. The working group is not likely to come to a resolution prior to the first draft, however.

A note will be added to the core media type section that Opus is still under consideration and will be resolved in a future draft.