If you want to share an MP4 animation with sound on FurAffinity, the modern solution or workaround is to make an FA Gallery submission with a still thumbnail from the video and use a link-to-file hoster like DropBox, GoogleDrive or Mega to host the animation file itself, then provide a link in the still-image thumbnail submission's description to the video at the hoster you've chosen. A lot of flash animators whose work I've enjoyed have been doing this with their grandfathered-in SWF emulated submissions for a while- usually much higher quality in MP4 format than their posted Flash SWFs- and more recent animators' postings have used the method I've mentioned specifically.
-2Paw.