Fixing a backlog
To get rid of a big backlog of speech that has built up:
1) Stop the timer or timers that are causing the backlog of speech.
2) Restart the app.
Your timers should continue counting down, but any speech that was queued to speak will be cleared.
Preventing a backlog
To prevent a backlog of speech, you can either:
1) Add less frequent speech to timers, or have fewer timers with speech running at once
and/or
2) Increase the rate of speech in settings from within the app.