<prosody rate="slow"> this is speaking slowly </prosody>.
<prosody rate="fast"> this is speaking fast </prosody>.
<prosody rate="-50%"> this is 50% slower </prosody>.
Precise pauses can also be inserted using the <break/> tag, e.g.
Break for 100 milliseconds <Break time="100ms"/> Okay, keep going."
Break for 3 seconds <Break time="3s"/> Okay, keep going."
Voices and languages can be intermixed using the <voice> tag, e.g.
<voice name="crystal">Crystal, 1 2 3.</voice>
<voice name="mike">Mike.
<voice name="rosa">Rosa, 1 2 3.</voice>
Back to Mike.</voice>