Support both @src and <source> child of <audio> (embedded resources)? #114
Description
While working on the specification for adding audio recordings I reminded myself of the various ways in which an audio recording can be embedded and referenced, of which there are at least 5 in total. Requirement R15 of [DAPT](https://www.w3.org/TR/dapt-reqs/#requirements) is clear that both referenced and embedded options need to be available, but should we be syntactically restricting the options for each? Will raise as separate issues.
Originally posted by @nigelmegitt in #105 (comment)
Given some embedded audio resources:
<head>
<resources>
<audio xml:id="audioRecording1" type="audio/wave">
<source>
<data>[base64 encoded audio data]</data>
</source>
</audio>
<data xml:id="audioRecording2" type="audio/wave">
[base64 encoded audio data]
</data>
</resources>
</head>The following two options exist in TTML2 for referencing embedded audio resources:
srcattribute in<audio>element referencing embedded<audio>or<data>:
<audio src="#audioRecording1"/>
...
<audio src="#audioRecording2"/><source>element child of<audio>element.
<audio>
<source src="#audioRecording1"/>
</audio>This second option has an additional possibility of specifying a format attribute in case type is inadequate. It also permits multiple <source> child elements, though it is unclear what the semantic is intended to be if multiple resources are specified - presumably, the implementation gets to choose one somehow.