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)

The following two options exist in TTML2 for referencing external audio resources:

1. src attribute in <audio> element.

<audio src="https://example.com/audio_recording.wav" type="audio/wave"/>

2. <source> element child of <audio> element.

<audio>
    <source src="https://example.com/audio_recording.wav" type="audio/wave"/>
</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, and we specify that in this case the implementation must choose no more than one.

[Edited 2023-03-29 to account for the "play no more than one" constraint added after the issue was opened]