Fix the support for phonemes

[jefer94]

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A screenshot that you have tested with "Try this API".

Link to the code that reproduces this issue. A link to a public Github Repository or gist with a minimal reproduction.

https://github.com/jefer94/gcloud-resize

A step-by-step description of how to reproduce the issue, based on the linked reproduction.

<speak>
      <phoneme alphabet="ipa" ph="soso"></phoneme>
</speak>

1. Go to https://console.cloud.google.com/speech/text-to-speech
2. Write that input on SSML
3. Click play button, each time you will heart nothing

A clear and concise description of what the bug is, and what you expected to happen.

In this case, reproduce /soso/ but really I want Text to Speech can reproduce all phonemes, right now it only support a small IPA subset

A clear and concise description WHY you expect this behavior, i.e., was it a recent change, there is documentation that points to this behavior, etc. **

Because I wrote an IPA reader, and if Text to Speech doesn't support it, I can't use Text to Speech

[github-actions]

Issue was opened with an invalid reproduction link. Please make sure the repository is a valid, publicly-accessible github repository, and make sure the url is complete (example: https://github.com/googleapis/google-cloud-node)