Opus is tricky, it's royalty free but it's not a "free (as in free software)" standard, it's based on Patents held by the likes of Broadcom and MSFT which make them available without royalties.
This is what I've mentioned earlier there are quite a few royalty free but they are still based on held patents which are offered without royalties.
That said honestly I'm not sure how big royalties are HDMI has quite steep royalties DisplayPort doesn't and with pretty much all devices that come with a single/mono-cultured display interface will use HDMI rather than DisplayPort.
Do royalty free patents stop something from being free software? That would be surprising.
Opus' patents have patent retaliation clauses. It would be funny if those were incompatible with the GPL, because the GPL is essentially a "copyright retaliation" license.
This is what I've mentioned earlier there are quite a few royalty free but they are still based on held patents which are offered without royalties.
That said honestly I'm not sure how big royalties are HDMI has quite steep royalties DisplayPort doesn't and with pretty much all devices that come with a single/mono-cultured display interface will use HDMI rather than DisplayPort.