I can reproduce on my end as well. It might possibly be a regression of #0013034

I have same issue.

Hey guys, spent an hour or two trying to nail this issue down some more, here are my findings:

When you remove all your .ogg files, the mission is downloadable from Steam again.
The number of files or size of the mission doesn't seem to have an influence on this.
Changing the file-extension from *.ogg to something else (e.g. *.sqf) still causes the issue.
Converting your .ogg files to .mp3 causes the same issue.

It is my guestimate that Steam is blocking all missions with an audio-file in it, as to cover themselves from possible copyright-infringment lawsuits caused by idiot people uploading missions with copyrighted music in them. This is too bad for us that just use the custom sound features for voice-acting and the like.

Here is my mission on which I tested all this, see the change-notes:
http://steamcommunity.com/sharedfiles/filedetails/?id=178547195

Here are some other feedback tickets with the same issue:
http://feedback.arma3.com/view.php?id=14440
http://feedback.arma3.com/view.php?id=13034

So, after testing (see http://feedback.arma3.com/view.php?id=14440) there appear to be multiple limitations that have been placed since the 12th of September or thereabouts:

Limitation 1: No audio files in the missions.
Limitation 2: No missions above 2MB.