Fix audio-caption SC tag

[WilcoFiers]

Closes #686

[chaals]

Don't the tests also apply to 1.2.2, 1.2.4, and 1.2.9?

(I am not sure if I misunderstood the tag system though)

[WilcoFiers]

@chaals I don't think it does. Those are video SCs, this is audio only.

[chaals]

But they require, for video which includes audio (or for live-streamed audio in the case of 1.2.9 I think), audio captions - i.e. the same requirement.

(if they do apply, am I making the right assumption that multiple checkpoint tags should be applied?)

[WilcoFiers]

The audio-caption rule only checks <audio> elements. I don't see how 1.2.2 says:

all prerecorded audio content in synchronized media

There's a slight assumption we are making that the <audio> element isn't synchronised with something else on the page, but I think that's generally not the case. So 99% of the time I would say this only applies to 1.2.1. Also as a rule of thumb, we use one SC, not all that may be applicable.