There is a good article in the Joomla! docs about editing your own version of the views.
Copy the views you want to edit to your template and they will overide the default views.
For example copy components/com_mymuse/views/product/tmpl/product.php to templates/YOUR_TEMPLATE_NAME/html/com_mymuse/product/product.php.
You may have to create the middle directories templates/YOUR_TEMPLATE_NAME/html/com_mymuse/product/
Now edit as you wish. Even if you upgrade MyMuse, your views will not be overwritten.
For the Audio_HTML5 player:
Besides css and button images included in plugins/mymuse/audio_html5/skin, it uses the jquery ui css.
The background is contained in in a class called .ui-widget-content.
You could override this by placing some css in the file plugins/mymuse/audio_html5/skin/jplayer.blue.monday.css
It gets loaded after the jquery-ui.css.
Example: Background of the player
Your style could modify the ui style which is
border: 1px solid #AAA;
background: url('images/ui-bg_flat_75_ffffff_40x100.png') repeat-x scroll 50% 50% #FFF;
For the Cart:
Since versin 3.3.0, the cart has been divided into sections. It is quite easy to change the order of the sections. You may want to copy the file
/components/com_mymuse/views/cart/tmpl/cart.php to /templates/YOUR_TEMPLATE_NAME/html/com_mymuse/cart/cart.php
The file looks like this:
* @version $Id$
* @package mymuse
* @copyright Copyright © 2019 - Arboreta Internet Services - All rights reserved.
* @license GNU/GPL
* @author Gordon Fisch
* @website http://www.joomlamymuse.com
// no direct access
defined('_JEXEC') or die('Restricted access');
//To change the order of things, copy this to /templates/YOUR_TEMPLATE/html/com_mymuse/cart/cart.php and edit.
// To edit the parts, copy the part, ex: /templates/YOUR_TEMPLATE/html/com_mymuse/cart/cart_order_summary.php and edit.