@stephann that's not quite correct, you're not converting to a mp4 file with no audio, you're converting to a mp4 file with mp2 audio which most mastodon servers likely won't have installed due to license reasons, so they fail to convert it on upload and just leave out the audio. (if they do have it installed, you'll have audio though!) Instead of "-acodec mp2" use "-an" to simply not transcode the audio, this will also run faster and be a smaller file.