Please note that the Juicebox module for Drupal is an unofficial plugin which was not written by ourselves (but is well supported by its author in the Drupal forum). This is noted on the module's support page:

Upgrading the Drupal module to Juicebox-Pro does not change the module's interface.
There are no individual controls for each Pro configuration option. Instead, you can add Pro configuration options as text (for example, autoPlayOnLoad="TRUE") into the 'Juicebox Library - Pro / Manual Config' section of your Drupal Dashboard ('Home -> Administration -> Structure -> Content types -> Article -> Manage Display -> Juicebox settings').
Here is a Drupal forum post with the author's thoughts on adding further options to the module's interface.

This sounds like there may be a CSS conflict between your current theme and the your Juicebox gallery. Some CSS rules from your theme might be overriding the gallery's own CSS rules and adversely affecting the gallery's layout. If your theme uses global CSS rules which, for example, apply to all <div> or <img> tags on your web page, then they will also apply to these elements within the gallery. It is not possible to isolate a Juicebox gallery (or any other HTML element) from global CSS rules and the gallery will have no option but to inherit them. The best solution would be to apply CSS rules to only those elements on your web page that require them through use of CSS selectors (classes and ids). However, this might be difficult with a theme that you did not write yourself.
You might like to at least test this theory by using a different theme (at least for testing purposes) to see if it makes a difference.

You can also use your browser's developer tools (usually accessible via F12) to investigate which CSS rules are being applied to which HTML elements on your web page.
Please see this forum post for links to pages which might help if using a browser's developer tools.

Watermarking is a feature of JuiceboxBuilder-Pro (the desktop application which comes with Juicebox-Pro) only. Please see the Adding Watermarks support section for details.
Images are watermarked at the time the gallery is built by the application (at the same time the images are resized for the gallery). The watermarks are not added at the time the gallery is displayed so there are no watermark options which can achieve this.
You could perhaps use JuiceboxBuilder-Pro to watermark your images and then upload the watermarked images for use in your Juicebox gallery within Drupal.

Please see the Adding Audio support section for details.
You would need to point towards MP3 and OGG versions of an audio track using the audioUrlMp3 and audioUrlOgg Pro configuration options.
You could also set playAudioOnLoad="TRUE" (to automatically start the audio track playing when the gallery is initially loaded) and/or showAudioButton="TRUE" (to display the Audio Button on the Button Bar and allow visitors to your web site to toggle the audio on and off).

I hope these notes help to clarify things.

What you are describing is the Splash Page.
The Splash Page is a placeholder for the gallery which is displayed by default on small screen devices when the gallery is embedded in a page alongside other content (rather than displayed on a page of its own with dimensions of 100% x 100%, filling the browser window).
When the user clicks/taps the Splash Page, the gallery is expanded to fill the user's browser window.
For more information about the Splash Page and how Juicebox adapts to different devices and screen sizes, please see here.

You can choose to not use the Splash Page by setting showSplashPage="NEVER".
Alternatively, you can force the gallery to be displayed in Large Screen Mode (which, by default, does not use the Splash Page) on all devices and in all browsers by setting screenMode="LARGE".

If you choose to continue to use the Splash Page, you can customize it using the Splash Page configuration options.

A browser's developer tools can be very useful but they are not always the most accurate way to emulate a mobile device and determine how a gallery will look on such a device. (Just something to bear in mind when testing your page's layout.)

Please see this FAQ:
My Juicebox gallery shows too much space above or below the main image, how do I fix this?

Please see this note regarding Using Percentage Heights.

When setting a percentage height, make sure that all parent containers of your gallery have heights set via CSS. Otherwise, Juicebox may not be able to determine what its actual height should be. For example, if you set a gallery height of 100%, Juicebox needs to know what its height should be 100% of. In a CMS environment (such as Drupal) where you may have little or no control over the containers that the gallery is nested within, it might be wise to use a fixed pixel height (such as '600px') instead of a percentage to avoid problems that can arise when using a percentage.

If your current theme does not already so so, you could try scaling your web page for mobile devices but adding the following tag to the head section of your web page. This might help.

<meta name="viewport" id="jb-viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1, maximum-scale=1, user-scalable=0" />

Set showAutoPlayButton="TRUE" and the AutoPlay Button will de displayed in the Button Bar and the user can click/tap this to toggle AutoPlay on and off.

I hope this information helps.