If you'd like your Audio Button to stand out from the other icons on the Button Bar, you could color it differently by using CSS such as the following. (This example will color the Audio Button red.)
Put this CSS code at the foot of your gallery's 'jbcore/classic/theme.css' file. (If you put the code in the gallery's embedding page, then the Audio Button will not be colored if/when the gallery is expanded in a new page (for example, on iOS devices).)
This might at least attract some attention and users will hover over the button where they will then see the rollover tooltip text "Play Audio". If you like, you can change this tooltip text via the languageList configuration option. Please see the International Gallery Text support section for details. (The languageList option was intended for translating the on-screen text into different languages but you can use it to change the messages to whatever you like.)
If you want to actually change the Audio Button icon itself, then this is also possible (but more difficult).
Please check out the Using Custom Icons support section in the Theming Guide for details.
Alternatively, you could create your own Audio Button and place it alongside the gallery on the embedding page.
Take a look at the Juicebox-Pro API demo gallery here. You'll see that there is a 'Toggle Audio' button to the left of the gallery. You could do something similar for your own gallery's web page. (Check out the source of the demo in your browser to see how it is done.)
Your custom Audio Button does not need to be an HTML button, though. You could make any HTML element (e.g. text or an image) clickable and act like a button.
I hope this gives you some ideas as to how you could make your Audio Button more prominent now that autoplay is becoming less likely to happen (depending on the user's browser).