Audio and video seem to me very similar words by usage. I often hear the plural form for video, but is there a plural form for audio? Can I say audios? I've never heard it being used.
Yes, you can. Both audios and videos took off in the 1980s, but videos gained more traction than audios, probably because there were already well-established short words to talk about audio recordings (records, 8-tracks, tapes) when video recordings became available on similar recording mediums. However, audios is being used legitimately in print and I doubt anyone would be confused as to the meaning if you used it. So if you like it, use it.
When you hear a phrase like
both audio and video are being used as uncountable nouns.
So you can't generalise from that context to using the word audios, because the pluralisation would mean that audio is a countable noun.
You can, however, use video as a countable noun. This has come about through typical English speaker laziness: video is short for either videotape or videocassette and has come to refer to the single instance contained upon the tape that the video is recoded on.
Typically, to refer to a single instance of audio, you say
and in plural
Audios is not a word in common usage, so it is not advisable to use it.
Unlike video, audio recording media tend to be shortened to the specific medium, e.g.