~ is often used as a delimiter for <s>, <sup>, or <sub>, so I expect that would be problematic.
# is already in use for headings, so I’d say # is out because there are many headings that are also links.
^[]() might work (I’ve seen ^ used for <sup> so same problem as ~)
@[]() might work as most usages of @ are mentions or email addresses, neither of which would conflict with this syntax.
%[]() would probably work
|[]() might work (| is used in tables and I’ve proposed using it for <aside>, so maybe not).
{[]() and }[]() might work but I think they’d look weird and conflict with the proposal for an attribute syntax
+[]() might conflict with links in lists, or at least cause confusion
=[]() might work
&[]() looks silly to me.
If I had to pick, I’d probably go with ^[]() for video, and @[]() for audio. To me they seem to match the pattern of incorporating the first letter into the symbol (upside-down i = !, upside down v = ^, a = @).