Stories with page background audio continuously plays and overlaps

### Description

There are a few issues with Web Stories (v1.13.0) that have a page background audio:
- The audio from the previous story carries into the next story
- The audio continues to play after exiting the story
- The audio overlaps one another

Note:
- Issue occurs when using the Circle Carousel widget or the Web Stories block.
- Visit this site and navigate through the stories to see an example of the issue https://testsite.cmsdevrel.com/about
- Each story has audio on each page
- related Web Stories ticket: https://github.com/google/web-stories-wp/issues/9621
https://user-images.githubusercontent.com/66372350/139908468-1b7f140a-7f22-4dac-8b60-ddabf05f5987.mov

### Reproduction Steps

1. Let [Story 1](https://testsite.cmsdevrel.com/web-stories/ztest-3/) play through on the widget/block, and note audio carries through Story 2. [Story 2](https://testsite.cmsdevrel.com/web-stories/test-audio-2/) has its own audio on each page.
2. Skip through each story, unmute, and note the audio overlaps.
3. Exit player notice story audio continues to play

### Relevant Logs

_No response_

### Browser(s) Affected

Chrome

### OS(s) Affected

_No response_

### Device(s) Affected

macOS

### AMP Version Affected

2.1.4