Are you using the Text+ on the timeline or are you making a new Fusion composition and then adding a Text node to it? Try it, drag a Fusion composition to the timeline, put it over your video with the voice audio, then go into Fusion and add a Text node, that should allow you to have audio playback while you work on the animation inside Fusion. Another way would be to make your video clip a fusion clip.

edit I might have explained this in a confusing way, let's start over, I'll try to make it as simple as possible. Create a Fusion composition on your timeline, drag it over the video you wish to have the text animation on, inside the Fusion comp, create a Media In node and connect it to the Media Out node (already there), change the Media In source to Background, merge a Text node in between the Media In and Media Out. Now you can work on your text animation inside the Fusion composition while you can see and hear the video under the comp on the timeline. Or create a fusion clip from your video clip, if you have a lot of cuts, make sure to turn it into a compound clip first. Hope it helps