HI Deanna,
Unfortunately not. There is no bulk or automated way to do this. Storyline actually creates this speech directly from the text when you type it in. From that point on, it is just a regular audio file.
So unfortunately that also means that editing it to a different voice requires you to go in to each text and choose that new voice.
Im not sure how many you have to do, but one tip I use is if I have cuepoints or assets on the slide which synchonise to the TTS audio, I make a copy of each of these audio files (you can use the MEDIA LIBRARY within Storyline to make this easier).
From there, I open the audio file up in an audio editor program (I use Adobe Audition).
This lets me place an actual "marker" at every cue point, so when I create my new audio file - even if I dont speak the language - I can see exactly where each cuepoint and/or onscreen synchonisation needs to occur.
Hope that helps!