https://www.w3.org/TR/dapt/#audio-recording includes the following (and related examples):

• If the Source is an embedded audio resource, the Source and Type properties are represented together by exactly one of:

1. A src attribute that is a fragment identifier that references either an <audio> element or a <data> element, where the referenced element is a child of /tt/head/resources and specifies a type attribute and the xml:id attribute used to reference it;
   This mechanism cannot be used if there is more than one Source.

2. A <source> child element with a src attribute that is a fragment identifier that references either an <audio> element or a <data> element, where the referenced element is a child of /tt/head/resources and specifies a type attribute and the xml:id attribute used to reference it;

3. ...

Yet this appears to conflict with https://www.w3.org/TR/2021/CR-ttml2-20210309/ which requires the src attribute of an audio entity to be an audio value expression:

If an <audio> value expression takes the form of a bare fragment identifier, then that fragment identifier must reference an audio element that is a child of a resources element in the enclosing document instance.

and requires the src attribute of a source entity to be a data value expression:

If a <data> value expression takes the form of a fragment identifier, then that fragment identifier must reference a data element that is a child of a resources element in the enclosing document instance.

It looks like this is related to #220 (and perhaps others)