I don't see how aarch64 EME could possibly be related to this test; it's likely another issue.
From a quick comparison of the $objdir/config.status of an i686 and an aarch64 build, I can see that MOZ_FFVPX is defined for i686, but not for aarch64. So that means that we'll always take the "flac not supported" branch here:
This causes canPlayType() to report that we don't support flac.
This also mean our software vpX decoding will be using libvpx, which doesn't utilize parallelism as well as ffvpx.