When you embed an audio file on your site, it creates the HTML5 <audio> tag. That tag causes browsers to automatically create an audio player – the player is inserted by your browser itself, not by WordPress – and how it looks depends on the browser someone is using to view your site. If you look at your posts in Firefox vs Chrome vs Safari you’ll see the player looks different in each one.
Currently there is no way to modify the appearance of that player. This is a restriction of HTML itself, not of WordPress, and you’ll run into this on any website where you embed audio files using the <audio> HTML tag.
Aside: Your Business Plan gives you access to 24/7 live chat support, including for CSS modifications to your site, so you don’t need to post in the public support forums for help.