By default there are 22 different voices that can be used.
You can use the speechsynthesis.VoiceURIAt() expression to get the names of the URI files. It is a 0 based index so put 0 or 1 or 5 etc in the brackets.
Then in your speak text action, in the VOICE URI field, enter the name of the URI file inside the "". Your text will be spoken using this voice file.
EDIT: Read too fast and didn't see the mobile part. Not sure what is available on mobile