King Canute

m4a files auto open and play when I download them, whereas
mp3 files do not. How can I disable the autoplay of m4a files?

Don't you think that the answer will strongly depend on the browser you are
using ?

Sorry, if this is the wrong group, but it seemed more relevant
than others I found.

This group is about the Windows XP OS. Are you using that OS ?

Than again, depending on the browser you are using there might be a
better-suited newsgroup. Like "mozilla.general" on the "new.mozilla.org"
newsserver for FireFox related questions.

Regards,
Rudy Wieser