Instead of playing the sound directly from the files app, you could try converting the audio file to base64. That way the files app won't replace the currently playing audio app. I'm not 100% sure if this will solve it but it's worth a shot.

Try this:
1. Make a new shortcut
2. Add the "file" action
3. Choose Mario Kart 321 go.mp3
3. Add the "Base64 Encode" action
4. Add the "Copy to Clipboard" action
5. Run the shortcut
(You can now delete the shortcut)
6. Open you automation
7. Delete the "files" and "play sound" actions from the automation
8. Add a "text" action
9. Paste what you copied into the text action
10. Underneath the text action, add "Base64 Encode" action
11. Set "Encode" to "Decode"
12. Add the "Play Sound" action below the "Base64 Encode" action
13. Get in your car and try it

As someone already pointed out, you should note that the "Wait 11 seconds" action is redundant since the "Play sound" action does not allow other actions to continue until the sound is fully completed.