"languageCode": "en-US"

You can select the language accent in which you want the output voice using this property.

You can choose from the below:

Arabic — ar-XA, Bengali (India) — bn-IN, Chinese (Hong Kong) — yue-HK, Czech (Czech Republic) — cs-CZ, Danish (Denmark) — da-DK, Dutch (Netherlands) — nl-NL, English (Australia) — en-AU, English (India) — en-IN, English (UK) — en-GB, English (US) — en-US, Filipino (Philippines) — fil-PH, Finnish (Finland) — fi-FI, French (Canada) — fr-CA, French (France) — fr-FR, German (Germany) — de-DE, Greek (Greece) — el-GR, Gujarati (India) — gu-IN, Hindi (India) — hi-IN, Hungarian (Hungary) — hu-HU, Indonesian (Indonesia) — id-ID, Italian (Italy) — it-IT, Japanese (Japan) — ja-JP, Kannada (India) — kn-IN, Korean (South Korea) — ko-KR, Malayalam (India) — ml-IN, Mandarin Chinese — cmn-CN, Norwegian (Norway) — cmn-TW, Polish (Poland) — nb-NO, Portuguese (Brazil) — pl-PL, Portuguese (Portugal) — pt-BR, Romanian (Romania) — pt-PT, Russian (Russia) — ro-RO, Slovak (Slovakia) — ru-RU, Spanish (Spain) — sk-SK, Swedish (Sweden) — es-ES, Tamil (India) — sv-SE

"ssmlGender": "FEMALE"

You can also choose the gender for the voice output.

You can choose from FEMALE and MALE.

"name": "en-US-Wavenet-G"

You can try out different variations on how the voice output can be with this property.
Please refer to Google documentation on voices

"audioEncoding": "MP3"

You can choose the audio encoding of your choice from the below mentioned.

1. LINEAR16 - Uncompressed 16-bit signed little-endian samples (Linear PCM).

2. MP3 - MP3 audio.

3. OGG_OPUS - Opus encoded audio wrapped in an ogg container.