Could you not just unpack the opus file into a pure uncrompressed .wav which I'm pretty sure all browsers will support. Or if you don't mind dropping legacy-IE

Could you not just unpack the opus file into a pure uncrompressed .wav which I'm pretty sure all browsers will support. Or if you don't mind dropping legacy-IE support, FLAC would be an option. Or would the extra bandwidth be enough to be a worry?