How to export as Matroska Audio?
You did not mention the codec/compressionm you want (or whether completely uncompressed like WAV). MKA can contain many different codecs including MP3, AAC, and FLAC. I do not believe there is an obvious, most popular codec choice for MKA. The existing audio/FLAC export preset saves as MKA if that is ok for you. Otherwise, you can choose any other say AAC. Then, go into Advanced mode and change Format to “mastroska” and save the file with a name ending with “.mka”