This is reported by BBC folks:

The main bug we've seen is that with the TTML2 tests that exercise speech, the calls to Web Speech API don't always result in speech being generated first time. The first or second call seems to result in nothing happening, but repeating the request again usually does work. The behaviour is intermittent.

Browser: Version 69.0.3497.100 (Official Build) (64-bit)
OS: Mac OS X 10.11.6

Tests affected: 
ttml2-prstn-audio-pitch-on-p.xml
ttml2-prstn-audio-pitch-on-span.xml
ttml2-prstn-audio-speak-on-p.xml
ttml2-prstn-audio-speak-on-span.xml
Page URL: https://bbc.github.io/Adhere/

Steps to reproduce:
1. Load page
2. Select local video and test ttml2 file
3. Press play

There should be 3 utterances in each test, in the first 10 seconds. 

The pitch test utterances should be, in order: "This sentence is spoken normally.", "This sentence should be deeper." and "This sentence should be higher.". 
The speak (rate) test utterances should be, in order: "This sentence is spoken normally.", "This sentence is spoken slowly." and "This sentence is spoken quickly."