Trying to keep elements centered

Thanks for the plugin. It is very useful. I have a problem where I want to add an audio player and make sure it stays centered in order to accommodate different size screens on mobile devices. I cannot seem to get it to do so. putting margin-left: auto; margin-right: auto; dont seem to work. It seems that as I resize the browser window the player does not move alaong with it. Any way of getting this to work?

Thank you

https://wordpress.org/plugins/facebook-tab-manager/