I feel like you could simplify this to:
Sub CheckSpeakingTimer_Tick()
Dim jo as JavaObject = Starter.TTS1
If jo.RunMethod("isSpeaking", Null) then
'still talking, so leave play button disabled and timer active
'(good spot to hook in a talk-time display update)
Else
PlayButton.Enabled = True
CheckSpeakingTimer.Enabled = False
'(good spot to hook in a finished-talking event)
End If
End Sub
assuming that you disable PlayButton and enable CheckSpeakingTimer when you first/next send text to TTS1.
If you're into brevity and just want the play button to automatically enable and disable itself, you could even:
Sub CheckSpeakingTimer_Tick()
Dim jo as JavaObject = Starter.TTS1
PlayButton.Enabled = (jo.RunMethod("isSpeaking", Null) = False)
End Sub