How do I change the look?

There is a good article in the Joomla! docs about editing your own version of the views.

http://docs.joomla.org/How_to_override_the_output_from_the_Joomla!_core

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.
 $document->addStyleSheet('http://code.jquery.com/ui/1.9.2/themes/base/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
.ui-widget-content {
    border: 1px solid #AAA;
    color: #222;
    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
* @author mail This email address is being protected from spambots. You need JavaScript enabled to view it.
* @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.

if(isset($this->checkout_header)){
echo $this->checkout_header;
}

if(isset($this->download_page)){
echo $this->download_page;
}

if(isset($this->shipping_form)){
echo $this->shipping_form;
}

if(isset($this->order_summary)){
echo $this->order_summary;
}

if(isset($this->cart_display)){
echo $this->cart_display;
}
if(isset($this->cart_coupon)){
echo $this->cart_coupon;
}
if(isset($this->shopper_info)){
echo $this->shopper_info;
}

if(isset($this->next_form)){
echo $this->next_form;
}

if(isset($this->thankyou_form)){
echo $this->thankyou_form;
}

if(isset($this->makepayment_form)){
echo $this->makepayment_form;
}

if(isset($this->payment_form)){
echo $this->payment_form;
}

if(isset($this->checkout_footer)){
echo $this->checkout_footer;
}