Fix universal media_player mute

[colindunn]

Description:

Related issue (if applicable): fixes #9461

Pull request in home-assistant.github.io with documentation (if applicable): does not affect documentation

Example entry for configuration.yaml (if applicable):

No configuration change

Checklist:

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

[homeassistant]

Hi @colindunn,

It seems you haven't yet signed a CLA. Please do so here.

Once you do that we will be able to review and accept this pull request.

Thanks!

[mention-bot]

@colindunn, thanks for your PR! By analyzing the history of the files in this pull request, we identified @rmkraus, @fabaff and @pvizeli to be potential reviewers.

[colindunn]

Sorry first commit here so not 100% sure of the requirements. Do I tick all of the boxes even if there are no New items involved?

[emlove]

Good catch! For posterity, the ABC function definition is here: https://github.com/home-assistant/home-assistant/blob/392588e51979ad8bc1a1f6f177354626500289cf/homeassistant/components/media_player/__init__.py#L621

You don't need to worry about the other checkboxes. They're mostly applicable to new platforms only, not small bug fixes.

[emlove]

Thanks for the contribution! 🎉