audio short code [gn_audio] not getting centred

Hi Support
Please assist in getting the shortcode [gn_audio] to be visible in center when used with a width of less than 100%. I would like to display in the center of the post where it is used. I tried aligning it center using the editor, also used a CSS align:center, nothing working.

The code i am using is:

[gn_audio url=”http://www.somesite.com/xxx.mp3″ width=”50%” class=”align: center”]

Please suggest a possible solution.

http://ww.wp.xz.cn/plugins/shortcodes-ultimate/