If you are trying to output a FCP X project as WebM format, you will find FCP X doesn’t include WebM as an export option. Thankfully, with a little workaround, you can still create WebM files from FCP X projects.
3 Aug 2010 You can specify multiple source files by using the element.
The command you posted worked. The two messages you posted are warnings, not fatal errors. By the way, I use this command to create WebM videos:
The post offers 2 ways for you to convert WebM to MP4: converting WebM to MP4 online/offline with online video converter and WebM to MP4 converter program.